To finish the handle of a knife I apply boiled linseed oil until it stops soaking into the wood. Then I let the handle sit for about a week in order to let the oil oxidize and harden into linoxyn. Linoxyn protects the wood from the inside as apposed to a varnish which creates a hard layer on the outside of the wood that can chip off leaving exposed wood and will eventually start to rot. After the first week, in which the linseed oil hardens, I'd suggest applying oil to the handle once or twice a month for a year. This will produce the most aesthetically pleasing outcome.
My blades are hand forged from high carbon steel, easy to sharpen but prone to rusting. To help prevent rusting it's best to keep the knife out of it's sheath when in storage for long periods of time, make sure to dry it off after using it, and keep the blade oiled. I keep my blades oiled with an olive oil beeswax finish that I make or food safe mineral oil. Being high carbon steel the blade will take on a natural patina over time. This is totally fine, in fact I think it adds a lovely uniqueness!