What tahun is Oliver Wood in when Harry is a first year?

5th year. Because when Harry is in 3rd year, it's Wood's last chance to win the quidditch world cup. Plus, in the book it describes him as a burly, fifth year.
"But wood turned out to be a person, a burly fifth tahun boy who came out of Flitwick's class looking confused."

He's in the same tahun as Percy so he would have been in his fifth tahun at hogwarts.