Actually General Motors still holds the Oldsmobile trademark and renewed it in April of last year. They don't have to sell cars to keep it, as they still sell parts related to the vehicles alongside merchandise that carries the brand's name. You still have to get a license from General Motors to feature the Oldsmobile 442 in your videogame for example.
General Motors LLC ("New GM") still holds five trademarks on OLDSMOBILE, but they're all for merch, not cars. Trademark classes are hub caps, apparel, catalogs, badge holders, and metal banks. The trademark for the automobile class, 74138034, was cancelled in 2016 for failure to file a declaration of use.