diff --git a/tests/test_structs.py b/tests/test_structs.py
index e2ee35a0..16d758e1 100644
--- a/tests/test_structs.py
+++ b/tests/test_structs.py
@@ -1,6 +1,7 @@
 from bionetgen.modelapi.structs import Action
 import pytest
-from bionetgen.modelapi.structs import ModelObj, Action
+from bionetgen.modelapi.structs import ModelObj, Observable, Action
+from bionetgen.modelapi.pattern import Pattern, Molecule
 
 
 def test_modelobj_setitem():
@@ -44,6 +45,26 @@ def test_modelobj_line_label_setter():
     assert obj.line_label == "[1, 2, 3]: "
 
 
+def test_observable_add_pattern():
+    # Test for "Molecules" type
+    obs_mol = Observable(name="obs_mol", otype="Molecules", patterns=[])
+    pat_mol = Pattern(molecules=[Molecule(name="A")])
+    pat_mol.MatchOnce = True  # Should remain True
+    obs_mol.add_pattern(pat_mol)
+    assert len(obs_mol.patterns) == 1
+    assert obs_mol.patterns[0] == pat_mol
+    assert obs_mol.patterns[0].MatchOnce is True
+
+    # Test for "Species" type
+    obs_spec = Observable(name="obs_spec", otype="Species", patterns=[])
+    pat_spec = Pattern(molecules=[Molecule(name="B")])
+    pat_spec.MatchOnce = True  # Should be set to False
+    obs_spec.add_pattern(pat_spec)
+    assert len(obs_spec.patterns) == 1
+    assert obs_spec.patterns[0] == pat_spec
+    assert obs_spec.patterns[0].MatchOnce is False
+
+
 def test_action_print_line_v2():
     action = Action("simulate", {"method": "ode", "t_end": 100, "n_steps": 100})
     # Test base print_line