Got it working.

(I did not understand the CRC algorithm used.)

smalltag/_formats/_ogg.py

@@ -198,10 +198,13 @@ class _Ogg(SmallTag, tinytag._Ogg):
     def _crc_checksum(self, data: bytes) -> int:
         generator_polynomial = 0x04c11db7
         crc = 0
+        bitmask = 0xffffffff
 
         for byte in data:
-            byte ^= crc
-            crc = byte % generator_polynomial
-
-        return crc
+            for i in range(8):
+                b = bitmask if byte & (1 << 7) != 0 else 0
+                divide = bitmask if (crc & (1 << 31)) != 0 else 0
+                crc = (crc << 1) ^ (generator_polynomial & (b ^ divide))
+                byte <<= 1
 
+        return crc & bitmask

smalltag/smalltag.py

@@ -15,7 +15,7 @@ UnsupportedFormatError = tinytag.UnsupportedFormatError
 class SmallTag(TinyTag):
     _file_extension_mapping: dict[tuple[str, ...], type[SmallTag]]= {
         (".mp1", ".mp2", ".mp3"): None,
-        ('.oga', '.ogg', '.opus', '.spx'): tinytag._Ogg,
+        ('.oga', '.ogg', '.opus', '.spx'): None,
         ('.wav',): tinytag._Wave,
         ('.flac',): tinytag._Flac,
         ('.wma',): tinytag._Wma,
@@ -116,7 +116,8 @@ class SmallTag(TinyTag):
 
 # import formats after SmallTag definition to avoid circular imports
 # (this solution is stupid, but I don't know anything better.)
-from ._formats import _ID3  # noqa
+from ._formats import _ID3, _Ogg # noqa
 
 SmallTag._file_extension_mapping[(".mp1", ".mp2", ".mp3")] = _ID3
+SmallTag._file_extension_mapping[('.oga', '.ogg', '.opus', '.spx')] = _Ogg