The first oil finishes happened by accident. It was way back in the days of feudal castles. The first oak interior trim was left unfinished. As time passed the wood began to take on a rich color from the gases and grease coming from cooking pots hung over a fire. This eventually resulted in beautiful, colorful, and protective finishes.
Now it’s an ambitious project. It uses applications of hot linseed oil with each application rubbed into the wood then allowed to dry. It is well worth the time, patience, and the endless rubbing. It takes months. The advantage between an oil finish and other finishes is that the oil finish is in the wood and not on it.
The centuries old procedure is to brush on a 50-50 mix of boiled linseed oil and turpentine then allow it to dry about an hour then wipe off the excess. Repeat the process every day or two until four or more coats of oil have been applied. From time to time, over a period of months and even years, the oiling process can be repeated to renew and preserve.
Another way is to mix boiled linseed oil to pure turpentine in the proportions of 2 parts oil to 1 part turpentine. This mixture can be applied hot or cold. The hot mixture will set faster.
You want to start with glassy smooth wood using 5/0 garnet sandpaper. Wipe it with a clean, moist, cloth then allowed to dry. Then use sandpaper or 00-grade steel wool to remove any small whiskers that have been raised by the moist cloth.
To rub, I like to use the heel of my hand. I keep rubbing until the oil has been absorbed then add more. I repeat until I feel that the wood can absorb no more.
If you want the finish darker then add a little dark red oil stain into the rubbing oil.
For a higher gloss, the surface can be rubbed down with FFF pumice and rubbing oil. For even higher gloss, follow up a few days later with a rottenstone and oil rub.
Good luck.