OAS cards provide the DTMF receivers required for MOBEX Executive (regardless of line type).
The OP hasn't specified which mobex they are trying to use (I made an assumption they were looking to use executive).

The OS7030/OS7100 and OS7200S (and I assume the OS7070) all come with builtin MOBEX DTMF receivers, but the OS7200 MP20 (the OS7200 MCP card doesn't support mobex) and OS7400 don't.