If this happens during the quest "the fallen" and odahviing just flies around dragonsreach
and doesn't land so that you can fight him, you can exit the great porch wait for about a
day and try again. If that still doesn't fix the bug try also leaving your follower
outside the great porch, waiting a day and trying again.

If this happens after the main quest is completed and odahviing just flies around and
doesn't attack your enemies then leaves. It is more than likely because there is another
dragon nearby. Whether that second dragon is aggro'd or not doesn't matter it can still
mess up odahviiing's ai, causing him to bug out.

In both cases these are possible fixes to this bug. I don't know if they work 100% of the
time though. Worst case scenario, if nothing seems to fix the bug reloading a previous
save is always advised.