diff --git a/clinica/iotools/converters/oasis_to_bids/oasis_to_bids.py b/clinica/iotools/converters/oasis_to_bids/oasis_to_bids.py
index 9f0a1808d..1500b433e 100644
--- a/clinica/iotools/converters/oasis_to_bids/oasis_to_bids.py
+++ b/clinica/iotools/converters/oasis_to_bids/oasis_to_bids.py
@@ -117,11 +117,6 @@ def _create_sessions_tsv(
             bids_ids=bids_ids,
         )
 
-        # todo : when tested add to create_sessions_dict bc specific to oasis1
-        for bids_id in bids_ids:
-            sessions_dict[bids_id]["M000"]["diagnosis"] = (
-                "AD" if sessions_dict[bids_id]["M000"]["diagnosis"] > 0 else "CN"
-            )
         write_sessions_tsv(bids_dir, sessions_dict)
 
         return sessions_dict
diff --git a/clinica/iotools/converters/oasis_to_bids/oasis_to_bids_utils.py b/clinica/iotools/converters/oasis_to_bids/oasis_to_bids_utils.py
index c30a5ef37..a7f78950e 100644
--- a/clinica/iotools/converters/oasis_to_bids/oasis_to_bids_utils.py
+++ b/clinica/iotools/converters/oasis_to_bids/oasis_to_bids_utils.py
@@ -58,6 +58,9 @@ def create_sessions_dict(
         # managed before
 
         result = result.loc[bids_ids]
+        result["diagnosis"] = result["diagnosis"].apply(
+            lambda x: "AD" if x > 0 else "CN"
+        )
         result["session_id"] = "ses-M000"
 
         for bids_id, row in result.iterrows():
diff --git a/test/unittests/iotools/converters/oasis_to_bids/test_oasis_to_bids_utils.py b/test/unittests/iotools/converters/oasis_to_bids/test_oasis_to_bids_utils.py
index d6fd4d7f0..4e716de95 100644
--- a/test/unittests/iotools/converters/oasis_to_bids/test_oasis_to_bids_utils.py
+++ b/test/unittests/iotools/converters/oasis_to_bids/test_oasis_to_bids_utils.py
@@ -111,7 +111,7 @@ def expected() -> dict:
                 "session_id": "ses-M000",
                 "cdr_global": 0,
                 "MMS": 29,
-                "diagnosis": 0,
+                "diagnosis": "CN",
             },
         },
         "sub-OASIS10002": {
@@ -119,7 +119,7 @@ def expected() -> dict:
                 "session_id": "ses-M000",
                 "cdr_global": 0,
                 "MMS": 29,
-                "diagnosis": 0,
+                "diagnosis": "CN",
             }
         },
     }