Yes of course you can fill your form from obc category in MHT CET if you haven't use your caste certificate before like in your board exams But you must fulfill these conditions

If anyone is OBC but he comes under the CL (creamy layer) Section then he will not come under the OBC category. They are equal to general unreserved Category.so your category must fall under non creamy layer of OBC category.

So you have to be OBC-NCL (non creamy layer) for filling category OBC in MHT CET

Be prepared with your OBC-NCL caste certificate.you must obtain your category certificate before filing form.

If there is option of OBC-CL and if you are OBC-CL then you have to choose that option. If the option is unavailable then you have to choose general Category if you fall under OBC-CL.

GOOD LUCK!!