Because oil paint dries by oxidation which requires air to reach through all the layers and varnish will seal the paint and prevent it from finishing the hardening process, you need to wait about six months before applying a final varnish.
Any varnish that is called ‘picture varnish’ or says it is suitable as a final varnish for oil paintings will work. If you need to show or sell the painting before it is fully dry, you can use a spray retouching varnish or Gamvar.
Retouching varnish is a varnish diluted with extra solvent. It is so thin that the painting can breathe rather than being sealed completely. Final picture varnish can be applied over retouching varnish when the paint has fully dried. Some painters only use retouching varnish.