How to clean the oil painting?

Unlike any other paintings, oil painting are sensitive and they collect dust and dirt all over time. Cleaning them requires more than wiping them with a damp cloth. It can be a little unusual but you cannot use water or paint thinner - you need to use your saliva - literally. Using a cotton swab, wet it a little. Then slowly dab it in the painting. Aim for cleaning a square inch a time. Keep a glass of water handy so you wont be running out of saliva.

If you do not want to apply this kind of method, you can also use bread. Make sure to choose a doughy bread. Cut a little from the center and use it like a sponge to wipe the painting. Clear the crumbs and your painting is good as new.

Thank you for your advice. Maybe I can try the second method.

Is the painting varnished?

make sure that you check your q tip frequently to see that no colors or varnish is lifted. The spit cleaning method is used at the winterthur conservation labs. They've abandoned the bread and the sliced potato trick many years ago because some breads have residual alcohols left that can mess with varnishes. Varnish removal may still be needed but that should be left in the hands of a pro who is experienced and INSURED (Above all else , do no harm).