It's usually a cylindrical canister, often with metal or plastic end caps. Because of its location near the tank, you'll likely need to work under the car to access it. Always be cautious when working near fuel lines and have a fire extinguisher nearby.
Before attempting to locate or replace the filter, it's strongly recommended to consult a repair manual specific to your 1991 Oldsmobile 88. This will give you precise diagrams and instructions, minimizing the risk of mistakes. A Haynes or Chilton manual are good options.