I work in Maryland on salary, plus comm. if I work less than 40 hours a week my pay is deducted, now if I work overtime I don't get paid for it. Is this legal?

Sounds like your employer wants it both ways. Are you exempt or nonexempt?
http://www.dol.gov/esa/regs/compliance/whd/fairpay/fs17a_overview.htm

I guess I would be exempt. They will pay us hourly when it suits the company. Do I have any legal grounds?

If they pay you on an hourly basis at all, they have most likely invalidated the exemption from minimum wage and overtime that they would most likely be entitled to claim for you.

Contact the Maryland Dept. of Labor to file a complaint.