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.
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.