Let's say Oden plays 70 games, averaging 12 points and 8 rebounds in 25 minutes. What do you do then? Are you going to sign him to a three-year deal?
If he plays well, the Celtics probably won't want to bid against whatever multi-year contract offer he is likely to get. If he plays poorly or is injured, the Celtics won't want to keep him. Either way, if you sign him to a one-year deal, he doesn't seem likely to be a Celtic beyond next season. The best-case scenario is signing him and flipping to a team desperate for big man depth at the deadline.