If an employer doesn’t pay a fine, then, like most government agencies, there are policies in place in terms of debt collection. This works like many other debts, in that an employer will receive formal notification of the delinquency, interest will be assessed, etc. Other measures can also be taken, such as referring the fine to a collections agency, credit reporting bureaus, etc.
Keep in mind, in some cases OSHA will work with an employer initially to set up a payment plan, depending on the specifics. If a company anticipates not being able to pay the full amount, this is one of those things to discuss during the informal conference or the contest process.