diff --git a/osc/conf.py b/osc/conf.py
index 07dfc0b9..779799e6 100644
--- a/osc/conf.py
+++ b/osc/conf.py
@@ -126,7 +126,9 @@ HttpHeader = NewType("HttpHeader", Tuple[str, str])
 class OscOptions(BaseModel):
     def __init__(self, *args, **kwargs):
         super().__init__(*args, **kwargs)
-        self.extra_fields = {}
+        self._allow_new_attributes = True
+        self._extra_fields = {}
+        self._allow_new_attributes = False
 
     # compat function with the config dict
     def _get_field_name(self, name):
@@ -145,7 +147,7 @@ class OscOptions(BaseModel):
         field_name = self._get_field_name(name)
 
         if field_name is None and not hasattr(self, name):
-            return self.extra_fields[name]
+            return self._extra_fields[name]
 
         field_name = field_name or name
         try:
@@ -158,7 +160,7 @@ class OscOptions(BaseModel):
         field_name = self._get_field_name(name)
 
         if field_name is None and not hasattr(self, name):
-            self.extra_fields[name] = value
+            self._extra_fields[name] = value
             return
 
         field_name = field_name or name
diff --git a/osc/util/models.py b/osc/util/models.py
index f0926000..125557ba 100644
--- a/osc/util/models.py
+++ b/osc/util/models.py
@@ -6,6 +6,7 @@ It works on python 3.6+.
 This module IS NOT a supported API, it is meant for osc internal use only.
 """
 
+import copy
 import inspect
 import sys
 import types
@@ -82,9 +83,6 @@ class Field(property):
         # a flag indicating, whether the default is a callable with lazy evalution
         self.default_is_lazy = callable(self.default)
 
-        # whether the field was set
-        self.is_set = False
-
         # the name of model's attribute associated with this field instance - set from the model
         self.name = None
 
@@ -205,8 +203,13 @@ class Field(property):
 
     def get(self, obj):
         try:
-            return getattr(obj, f"_{self.name}")
-        except AttributeError:
+            return obj._values[self.name]
+        except KeyError:
+            pass
+
+        try:
+            return obj._defaults[self.name]
+        except KeyError:
             pass
 
         if isinstance(self.default, FromParent):
@@ -217,18 +220,23 @@ class Field(property):
         if self.default is NotSet:
             raise RuntimeError(f"The field '{self.name}' has no default")
 
+        # make a deepcopy to avoid problems with mutable defaults
+        default = copy.deepcopy(self.default)
+
         # lazy evaluation of a factory function on first use
-        if callable(self.default):
-            self.default = self.default()
+        if callable(default):
+            default = default()
 
         # if this is a model field, convert dict to a model instance
-        if self.is_model and isinstance(self.default, dict):
-            new_value = self.origin_type()  # pylint: disable=not-callable
-            for k, v in self.default.items():
+        if self.is_model and isinstance(default, dict):
+            cls = self.origin_type
+            new_value = cls()  # pylint: disable=not-callable
+            for k, v in default.items():
                 setattr(new_value, k, v)
-            self.default = new_value
+            default = new_value
 
-        return self.default
+        obj._defaults[self.name] = default
+        return default
 
     def set(self, obj, value):
         # if this is a model field, convert dict to a model instance
@@ -239,8 +247,7 @@ class Field(property):
             value = new_value
 
         self.validate_type(value)
-        setattr(obj, f"_{self.name}", value)
-        self.is_set = True
+        obj._values[self.name] = value
 
 
 class ModelMeta(type):
@@ -280,7 +287,16 @@ class ModelMeta(type):
 class BaseModel(metaclass=ModelMeta):
     __fields__: Dict[str, Field]
 
+    def __setattr__(self, name, value):
+        if getattr(self, "_allow_new_attributes", True) or hasattr(self.__class__, name) or hasattr(self, name):
+            # allow setting properties - test if they exist in the class
+            # also allow setting existing attributes that were previously initialized via __dict__
+            return super().__setattr__(name, value)
+        raise AttributeError(f"Setting attribute '{self.__class__.__name__}.{name}' is not allowed")
+
     def __init__(self, **kwargs):
+        self._defaults = {}  # field defaults cached in field.get()
+        self._values = {}  # field values explicitly set after initializing the model
         self._parent = kwargs.pop("_parent", None)
 
         uninitialized_fields = []
@@ -306,16 +322,15 @@ class BaseModel(metaclass=ModelMeta):
         for name, field in self.__fields__.items():
             field.validate_type(getattr(self, name))
 
-    def dict(self, exclude_unset=False):
+        self._allow_new_attributes = False
+
+    def dict(self):
         result = {}
         for name, field in self.__fields__.items():
             if field.exclude:
                 continue
-            if exclude_unset and not field.is_set and field.is_optional:
-                # include only mandatory fields and optional fields that were set to an actual value
-                continue
             if field.is_model:
-                result[name] = getattr(self, name).dict(exclude_unset=exclude_unset)
+                result[name] = getattr(self, name).dict()
             else:
                 result[name] = getattr(self, name)
         return result
diff --git a/tests/test_build.py b/tests/test_build.py
index 18e8c227..99dd5044 100644
--- a/tests/test_build.py
+++ b/tests/test_build.py
@@ -1,4 +1,3 @@
-import importlib
 import unittest
 
 import osc.conf
@@ -8,12 +7,7 @@ from osc.oscerr import UserAbort
 
 class TestTrustedProjects(unittest.TestCase):
     def setUp(self):
-        # reset the global `config` in preparation for running the tests
-        importlib.reload(osc.conf)
-
-    def tearDown(self):
-        # reset the global `config` to avoid impacting tests from other classes
-        importlib.reload(osc.conf)
+        osc.conf.config = osc.conf.Options()
 
     def test_name(self):
         apiurl = "https://example.com"
diff --git a/tests/test_conf.py b/tests/test_conf.py
index 5d6a41d2..bb7cba38 100644
--- a/tests/test_conf.py
+++ b/tests/test_conf.py
@@ -1,4 +1,3 @@
-import importlib
 import os
 import shutil
 import tempfile
@@ -105,7 +104,6 @@ plugin-option = plugin-host-option
 
 class TestExampleConfig(unittest.TestCase):
     def setUp(self):
-        importlib.reload(osc.conf)
         self.tmpdir = tempfile.mkdtemp(prefix="osc_test_")
         self.oscrc = os.path.join(self.tmpdir, "oscrc")
         with open(self.oscrc, "w", encoding="utf-8") as f:
@@ -116,6 +114,9 @@ class TestExampleConfig(unittest.TestCase):
     def tearDown(self):
         shutil.rmtree(self.tmpdir)
 
+    def test_invalid_attribute(self):
+        self.assertRaises(AttributeError, setattr, self.config, "new_attribute", "123")
+
     def test_apiurl(self):
         self.assertEqual(self.config["apiurl"], "https://api.opensuse.org")
 
@@ -407,26 +408,19 @@ class TestExampleConfig(unittest.TestCase):
 
     def test_extra_fields(self):
         self.assertEqual(self.config["plugin-option"], "plugin-general-option")
-        self.assertEqual(self.config.extra_fields, {"plugin-option": "plugin-general-option"})
-
-        # write to an existing attribute instead of extra_fields
-        self.config.attrib = 123
-        self.assertEqual(self.config["attrib"], 123)
-        self.config["attrib"] = 456
-        self.assertEqual(self.config["attrib"], 456)
-        self.assertEqual(self.config.extra_fields, {"plugin-option": "plugin-general-option"})
+        self.assertEqual(self.config._extra_fields, {"plugin-option": "plugin-general-option"})
 
         self.config["new-option"] = "value"
         self.assertEqual(self.config["new-option"], "value")
-        self.assertEqual(self.config.extra_fields, {"plugin-option": "plugin-general-option", "new-option": "value"})
+        self.assertEqual(self.config._extra_fields, {"plugin-option": "plugin-general-option", "new-option": "value"})
 
         host_options = self.config["api_host_options"][self.config["apiurl"]]
         self.assertEqual(host_options["plugin-option"], "plugin-host-option")
-        self.assertEqual(host_options.extra_fields, {"plugin-option": "plugin-host-option"})
+        self.assertEqual(host_options._extra_fields, {"plugin-option": "plugin-host-option"})
 
         host_options["new-option"] = "value"
         self.assertEqual(host_options["new-option"], "value")
-        self.assertEqual(host_options.extra_fields, {"plugin-option": "plugin-host-option", "new-option": "value"})
+        self.assertEqual(host_options._extra_fields, {"plugin-option": "plugin-host-option", "new-option": "value"})
 
     def test_apiurl_aliases(self):
         expected = {"https://api.opensuse.org": "https://api.opensuse.org", "osc": "https://api.opensuse.org"}
@@ -485,6 +479,15 @@ class TestConf(unittest.TestCase):
         }
         osc.conf.write_initial_config(conffile, entries)
 
+    def test_api_host_options(self):
+        # test that instances do not share any references leaked from the defaults
+        conf1 = osc.conf.Options()
+        conf2 = osc.conf.Options()
+
+        self.assertNotEqual(conf1, conf2)
+        self.assertNotEqual(id(conf1), id(conf2))
+        self.assertNotEqual(id(conf1.api_host_options), id(conf2.api_host_options))
+
 
 if __name__ == "__main__":
     unittest.main()
diff --git a/tests/test_grabber.py b/tests/test_grabber.py
index f88cd872..25493b83 100644
--- a/tests/test_grabber.py
+++ b/tests/test_grabber.py
@@ -1,4 +1,3 @@
-import importlib
 import os
 import tempfile
 import unittest
@@ -13,14 +12,10 @@ FIXTURES_DIR = os.path.join(os.path.dirname(__file__), "conf_fixtures")
 class TestMirrorGroup(unittest.TestCase):
     def setUp(self):
         self.tmpdir = tempfile.mkdtemp(prefix='osc_test')
-        # reset the global `config` in preparation for running the tests
-        importlib.reload(osc.conf)
         oscrc = os.path.join(self._get_fixtures_dir(), "oscrc")
         osc.conf.get_config(override_conffile=oscrc, override_no_keyring=True)
 
     def tearDown(self):
-        # reset the global `config` to avoid impacting tests from other classes
-        importlib.reload(osc.conf)
         try:
             shutil.rmtree(self.tmpdir)
         except:
diff --git a/tests/test_models.py b/tests/test_models.py
index e3d6895e..ccde8a4b 100644
--- a/tests/test_models.py
+++ b/tests/test_models.py
@@ -24,16 +24,21 @@ class TestNotSet(unittest.TestCase):
 
 
 class Test(unittest.TestCase):
-    def test_modified(self):
+    def test_dict(self):
+        class TestSubmodel(BaseModel):
+            text: str = Field(default="default")
+
         class TestModel(BaseModel):
             a: str = Field(default="default")
             b: Optional[str] = Field(default=None)
+            sub: Optional[List[TestSubmodel]] = Field(default=None)
 
         m = TestModel()
-        self.assertEqual(m.dict(exclude_unset=True), {"a": "default"})
+        self.assertEqual(m.dict(), {"a": "default", "b": None, "sub": None})
 
-        m = TestModel(b=None)
-        self.assertEqual(m.dict(exclude_unset=True), {"a": "default", "b": None})
+        m.b = "B"
+        m.sub = [{"text": "one"}, {"text": "two"}]
+        self.assertEqual(m.dict(), {"a": "default", "b": "B", "sub": [{"text": "one"}, {"text": "two"}]})
 
     def test_unknown_fields(self):
         class TestModel(BaseModel):
@@ -79,11 +84,12 @@ class Test(unittest.TestCase):
 
         m = TestModel()
 
-        field = m.__fields__["field"]
-        self.assertEqual(field.is_set, False)
+        self.assertNotIn("field", m._values)
         self.assertEqual(m.field, None)
+
         m.field = "text"
-        self.assertEqual(field.is_set, True)
+
+        self.assertIn("field", m._values)
         self.assertEqual(m.field, "text")
 
     def test_str(self):
@@ -95,7 +101,6 @@ class Test(unittest.TestCase):
         field = m.__fields__["field"]
         self.assertEqual(field.is_model, False)
         self.assertEqual(field.is_optional, False)
-        self.assertEqual(field.is_set, False)
         self.assertEqual(field.origin_type, str)
 
         self.assertEqual(m.field, "default")
@@ -111,7 +116,6 @@ class Test(unittest.TestCase):
         field = m.__fields__["field"]
         self.assertEqual(field.is_model, False)
         self.assertEqual(field.is_optional, True)
-        self.assertEqual(field.is_set, False)
         self.assertEqual(field.origin_type, str)
 
         self.assertEqual(m.field, None)
diff --git a/tests/test_output.py b/tests/test_output.py
index 8c02adbc..1b7337ec 100644
--- a/tests/test_output.py
+++ b/tests/test_output.py
@@ -1,5 +1,4 @@
 import contextlib
-import importlib
 import io
 import unittest
 
@@ -74,12 +73,7 @@ Key    : Value
 
 class TestPrintMsg(unittest.TestCase):
     def setUp(self):
-        # reset the global `config` in preparation for running the tests
-        importlib.reload(osc.conf)
-
-    def tearDown(self):
-        # reset the global `config` to avoid impacting tests from other classes
-        importlib.reload(osc.conf)
+        osc.conf.config = osc.conf.Options()
 
     def test_debug(self):
         osc.conf.config["debug"] = False
diff --git a/tests/test_vc.py b/tests/test_vc.py
index 491a677a..9da14b55 100644
--- a/tests/test_vc.py
+++ b/tests/test_vc.py
@@ -1,4 +1,3 @@
-import importlib
 import os
 import unittest
 
@@ -11,7 +10,7 @@ from .common import patch
 
 class TestVC(unittest.TestCase):
     def setUp(self):
-        importlib.reload(osc.conf)
+        osc.conf.config = osc.conf.Options()
 
         config = osc.conf.config
         host_options = osc.conf.HostOptions(
@@ -21,8 +20,6 @@ class TestVC(unittest.TestCase):
         config["apiurl"] = host_options["apiurl"]
         self.host_options = host_options
 
-    def tearDown(self):
-        importlib.reload(osc.conf)
 
     @patch.dict(os.environ, {}, clear=True)
     def test_vc_export_env_conf(self):