Question: I signed a contract with my employer stating that for any and all reasons I am not working at that company before 2 years, I have to pay back x amount of dollars for the cost of training. Now this wasn't a small sum of money, but I got fired for reasons unknown (lack of "performance"). Is this common practice (and/or legal) in Ohio or elsewhere?

Answer: 
This site cannot speak to whether or not this is a legal practice.

I do know of at least one time this has happened in Ohio -- a person was let go, changed jobs, and the person's original employer wanted the person (or new employer) to pay for the training.

Asking employees to pay for training when they voluntarily leave is one thing, but it makes no sense when the company releases an employee.

Agreed that a brief meeting with a lawyer would be valuable. The lawyer would be able to advise on the legality and enforceability, which are often two very different things.

Employers sometimes do put essentially illegal clauses in contracts, since they know the employee would have to go to court and pay legal fees to get a judgement in their favor. Often it's easier/cheaper for the employee to go along with a contract clause than try to fight it. And very few want to be the person who sued their last employer.
A regular search, <https://www.google.com/?ion=1&espv=2#q=recoup+training+costs+employee+after+firing+them+ohio> might help you a little bit.  

It appears that the answer is - it depends.  

Most of the info is directed at employees who leave voluntarily. 

Did you know that a fired employee can sue for wrongful termination? I've seen employers agree to lots of things to avoid that suit. Suits cost a lot of money, especially that one. They may be willing to agree to not be sued and not sue you. This is especially true if it is a large or tiny company. 

Regardless of the wording of the question - including the legality part, it is best to get an employment attorney in the state in question.