Otopeni's dining scene is shaped almost entirely by the airport. The terminals have the usual range of fast food, coffee chains, and sit-down restaurants oriented toward transit passengers — prices are noticeably higher than in Bucharest, and the quality is consistent with airport dining anywhere in Europe. For a proper meal at a fair price, the train to Bucharest takes 25 minutes and puts you in reach of the full range of Romanian and international cuisine in the capital.
If you are staying in the hotel zone near the airport (Novotel, Crowne Plaza), both have restaurants that are decent if unremarkable. There are a handful of local Romanian restaurants in Otopeni town itself, but they have no particular reason to visit if Bucharest is accessible.