I used Osmo UV Protection Oil on our new porch in July 2009. Unfortunately I did not reapply as recommended by Ian since then and the Oak has discoloured significantly. I think it started to fail within about 12 months, so it may need to be reapplied more frequently.
I wonder if there is some way of reviving the oak to it's former glory?
There is a way to revive it, Osmo do a product called Wood Reviver. We tried it on some oak that had gone grey and it really brought it back. Not to the original colur but we were very impressed. You brush it on and start scrubbing...after a while the grey gel starts coming off and then you wash down with water prior to overcoating.