Is the OMR 500 given by a usurious bank as a prize considered usury, especially since there are no Islamic banks in the country and no competition was entered?

It is not permissible to deposit money in an interest-based (usurious) bank in the first place, whether with or without interest. The prize you received for depositing your money without interest (current account) is a gift from the bank and not usury. The ruling on accepting it falls under the issue of dealing with someone whose wealth is mostly from illicit sources.