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 interest will be assessed, you’ll get notifications, and then other measures can be taken, such as referring it to a collections agency, credit reporting bureaus and so on.
Keep in mind, though, that in some cases OSHA will work with you initially to set up a payment plan, depending on the specifics. So, 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.