Im an OEC-SC category student.Scince there is no such option as OEC in the application of NEET, which category should I choose in the B NEET application?

OEC Means Other Eligible Communities which is a state level categorisation. Sometimes OEC Category is grouped into furthur various categories like Scheduled Caste(SC) and Scheduled Tribe(ST). Now Neet is a national level examination so you'll not find OEC category here. As you told you belong to OEC Scheduled Caste so you can choose Scheduled Caste in application or counselling form of Neet. OEC SC and SC comes with almost same reservation benefits so you don't need to think about it much, you can choose SC category in national levels.