diff --git a/clinica/iotools/bids_utils.py b/clinica/iotools/bids_utils.py
index de47302ad..738882d1b 100644
--- a/clinica/iotools/bids_utils.py
+++ b/clinica/iotools/bids_utils.py
@@ -242,7 +242,7 @@ def from_original_study_id(cls, study_id: str) -> str:
         )
 
     def to_original_study_id(self) -> str:
-        return "OAS1" + self.split("OASIS1")[1] + "MR1"
+        return f"OAS1_{self.split('OASIS1')[1]}_MR1"
 
 
 class OASIS3BIDSSubjectID(BIDSSubjectID):
diff --git a/test/unittests/iotools/test_bids_utils.py b/test/unittests/iotools/test_bids_utils.py
index 6fe0ca75d..65ef601ed 100644
--- a/test/unittests/iotools/test_bids_utils.py
+++ b/test/unittests/iotools/test_bids_utils.py
@@ -102,8 +102,7 @@ def test_study_to_bids_id_value_error(study, study_id):
         (StudyName.GENFI, "MAPT009", "sub-MAPT009"),
         (StudyName.OASIS3, "OAS30001", "sub-OAS30001"),
         (StudyName.HABS, "P_INIBUB", "sub-HABSINIBUB"),
-        # (StudyName.OASIS, "OAS1_0001_MR1", "sub-OASIS10001"),
-        # todo : check OASIS
+        (StudyName.OASIS, "OAS1_0001_MR1", "sub-OASIS10001"),
         (StudyName.IXI, "IXI001", "sub-IXI001"),
     ],
 )