Even before getting to the main point, bringing that money into USA as a traveler / visitor will be a hurdle as a threshold exceeding $10,000 must be declared upon entry. Will they allow the entire 5000 OMR stack in or would that be confiscated even when declared?
Like this: after paying for the food in $, suddenly giving them more money: I’m only a visitor (as I only have foreign currency, i.e. Omani Rial in cash) being like, “Here is 5000 OMR ($13,020) for your efforts” so despite that being a lot of money, will they take it?
Tipping after paying is normal. Tipping in foreign currency is not. If it’s that large of a tip, and you explain the conversion rate, I imagine most people would be thrilled. But if an unknown or average amount in OMR is the only tip you give at a restaurant, their first reaction may be anger, as they will be expecting an approximately 20% tip in US dollars.