Purchase a paint remover such as Durmond's Peel Away 7, which is formulated specifically for the removal of oil-based paints from wood or Klean Strip's Premium Stripper, also designed to remove oil-based paints without damaging wood.
Apply the stripper with an acrylic brush. Don't buy an expensive brush, as you will not be able to use it again after using it with paint remover. Brush on as thick a coat as you can on the oil-based paint with a single brush stroke. Brush over the paint in one direction. Multiple brush strokes will cause the stripper to begin to separate and work less well.
Let the remover sit on the paint for the full duration given on the product instructions, usually 10 to 15 minutes. Scrape the dried stripper off with a plastic scraper so as to not damage the wood underneath. You may need to use a little steel wool to remove small, hard-to-remove spots. If the paint you are removing is very thick, it may take several applications of remover to get all of the layers off.