Oak Bench Slats Roundover Long length

It is up to you and really depends what look you are after. They will go grey if you leave them and that is a natural barrier and is generally my choice with oak. Otherwise you can vanish/oil them and this will provide extra protection especially from moisture which is the worst thing to cause decay, but it will be an ongoing job with most oils. I have found with tung/danish oil that if left outside over winter as it will start to weather black and then grey in a matter of months and to avoid this it requires constant re coating.
 
We have recently starting stocking Restol which is able to protect the wood and maintain the oak for several years without a re coat and is proving very popular.