Hi Jon.
You have plenty of choice, depending on the durability you need. Danish Oil always looks good on oak, is easy to apply and reasonably durable. Also, it is easy to refresh with another coat if necessary.
An oil based polyurethane varnish is very hard wearing and also suits oak well. The snag is that it can look a bit like treacle if not applied well. My solution to this is to dilute it by about 40% with white spirit and apply lots of thin coats with a cloth, rather as you would with Danish oil. It takes longer but you can get a beautiful natural looking sheen which is worth the effort.
Shellac is also good but less durable. You can brush it on or apply with a cloth, building up the coats very quickly as, being alcohol based, it dries very quickly. Beginners are often scared off by shellac because it is the basis of french polishing with all the mystique that surrounds it, but it is, in fact very forgiving.
I would avoid water based finishes; in my experience these tend to look unnatural on oak, taking the warmth and life out of it.
Of these option, in your position I would go for the polyurethane; I have a brown oak coffee table I made about 15 years ago which I finished in this way and it still looks good.
Hope this helps,
Jim
