Oiling is the best way to highlight grain and raw oils, raw linseed oil and pure tung oil, can work well outdoors. But you need to top up the finish periodically if you want it to keep looking its best.
I presume you don't want a glossy varnished look or you'd have thought of spar varnish. But there's teak oil of course, specifically intended for use on hardwoods outdoors. It's made from varnish but heavily thinned so it gives more of an in-the-wood finish instead of a thick shiny surface film. Teak oil should protect the wood better than either linseed or tung oils but again needs regular reapplication.
Many of the more current finishes for exterior stuff, the "wood oils", are blends of oils, waxes and resins and promise long-term protection without the need for a too-frequent refinishing schedule but as a rule they don't make the wood look quite as good.
