OML Fault is generated when the OML is disconnected b/w BTS and BSC.Its a critical Alarm in Huawei.Although BTS reset automatically after 15 min if the issue persistent.u can also check these steps.
1).The transmission line is faulty,
2).The BTS has a power failure.
3).The TRX of the BTS is faulty.
For any query, feel free to ask.