Why is my 2002 4.2 l Chevy trailblazer misfiring after I changed the oil

Have the vehicle scanned for trouble codes to know which cylinder is miss firing. the look at possible causes like, Faulty spark plug (s), Faulty ignition coil (s), Clogged or faulty fuel injector (s), Intake air leak, Fuel injectors harness is open or shorted, Fuel Injectors circuit poor electrical connection, Ignition coils harness is open or shorted, Ignition coils circuit poor electrical connection, Insufficient cylinders compression, Incorrect fuel pressure.

It's a coincidence that it started after the oil change. If check engine light is on, go to Autozone for a scan to find bad cylinder. It's almost certainly a plug or coil. If a plug, probably change them all. if no check engine light, take it out and beat on it until the light turns on. Several 0-50 at full throttle usually will set the trouble light.