Let's figure out ability scores. Genius intellect translates to 17-18. Dex is probably decent with their archery skills. Con is probably average. Wis is probably a tad higher than Cha, and both lower than Int.
Shall we give them good darkvision since they are associated with darkness and the underworld?
Devloping an idea upthread...
Sylvan Archer (Ex): An oh-do-wa is treated as an elf for purposes of meeting the prerequisites of feats, prestige classes (such as arcane archer), and magic items related to archery.
Enhance Arrow (Su): Every nonmagical arrow an oh-do-wa nocks and lets fly becomes magical, gaining a +1 enhancement bonus. Unlike magic weapons created by normal means, the oh-do-wa need not spend experience points or gold pieces to accomplish this task. However, an oh-do-wa's magic arrows only function for that oh-do-wa. This ability stacks with the arcane archer's class feature of the same name (thus the enhancement bonus is +2 at 1st level, +3 at 3rd level, +4 at 5th level, +5 at 7th level, and +6 at 9th level).