As an MMA gym owner, can you still be liable even after making students sign waivers?

Yes.  A waiver, particualrly a form waiver which is not negotiated and in connection with which the customer had no opportunity to negotiate, can only go so far.  As a general rule (not sure if Ohio is exactly the same) you can waive claims for negligence, but not for recklessness and certainly not for intentional acts.  In other words, if the mats are so slippery form recent sweat that someone slips and falls on them, the waiver may bar a claim.  If you get angry and intentionally knock someone through a wall, it will not. There are obviously a lot of areas in the middle where the outcome is not certain.