Firstly, your database should be refined further. It doesn't make sense to have a column with just one data in it. either your DB holds both data (e.g. OPEC & Non-OPEC) and the DB table_name is good enough to asses that list has only OPEC Countries in it.

For your REST API, the most logical uri path would be /opec/member_countries for OPEC based

If you planning to make it extensible, then /org/{OPEC}/member-countries would be a better uri.