That happens to me all the time, you just gotta learn to watch for it and pinch the tubing when the siphon starts. Like the previous poster said though, it's VERY hard to oxidize your beer. You mainly don't wanna be splashing it around post-boil but pre-chilling. I've had all sorts of accidents happened with my beer splashing as I've transferred it and never really had an issue with oxidation. I think the only oxidized beer I've ever had was the first beer I ever did, a Nut Brown, after drinking it over a year later...it didn't taste too grand.