So many ideas...
This is entirely the DM's call... so many ways to play this....
For all gaming purposes, it could be ruled that the char is entirely human. For roleplay, either non distinguishable from other humans, or easily distinguishable (this would only enhance roleplay, and would be kinda a gip, as they would take racism penalties on something they don't benifit from directly in-game).
They could be given all the abilities of both (the completely half orc, half elf pairing, no human characteristics), which would be somewhat disturbing to see I think. Probably would have to be given a -1 lev adjustment or some other sort of negative (perhaps everyone in this world is so wildly repulsed by the character that all Roleplay suffers... but that would kind of suck).
My favorite idea so far is to take all the stats from Human, half-orc, and half-elf, put them into one giant list, and roll for random racial bonuses. You could end up with a +2 strength, -2 con char with immunity to sleep spells and +4 skill points at base level and +1 every additional level... or someone with +2 Dex and -2 Charisma, with darkvision, counts as an orc for all intents and purposes ingame, and an extra feat at the first level... I'd just make sure that it's random to prevent min-maxing.
[addendum: as some racial bonuses are obviously better than others, you'd have to either separate the bonuses into 'minor' and 'major' categories, or submit it for DM's approval]