In general, oil paintings are made to last the test of time (look at all the art works in the museums). In order to care for them to maintain their pristine look over the years, here are some things you can do.
1. Use a soft brush to brush off dust that might have accumulated.
2. If the mark is more stubborn, use a white cotton cloth soaked in water to softly rub over the surface.
Note: Be gentle with paintings with thick paint later, as you do not want to break hardened paint.