Baskets or sacks full of giant centipedes dropping from the ceiling comes immediately to mind.

Lacking wood (no pun intended), he could use the bones (still no pun intended, really) to fashion various traps, and sharpening them into stakes.

Too bad he doesn't have any rot grubs...

He could make a slimy, flammable paste with the bat guano, paint the walls, floor and ceiling in a room with it and somehow lure the party into the area before setting it alight.

Alternatively, he might coat his bone spike traps with the substance to cause infection and disease the wounds.

He might make a lasso trap to encircles a character's leg, before dragging that character into the leech infested pool...

Here are some ideas for hit-and-run attacks that should drive your players crazy:

1) From a safe distance, the invisible Oni hurls a rock into their midst with darkness cast on it.  While they blunder around blindly, he takes potshots with an oversized Japanese bow whose arrows are coated in toxic and paralyzing venoms.  When the PCs seem to be about to escape the darkness, he turns invisible and flies away.

2) The Oni dresses one of the ogre corpses to look just like himself and props it in a shadowy corridor while hiding behind it.  When the PCs turn the corner and see the fake Oni, the real Oni nails them with a cone of cold from behind the decoy.  He then flies away invisibly while the PCs spend a round wasting missiles and (hopefully) spells on the decoy.

3) Once the players are good and aggravated, and have burned most of their spells, they round a bend to find the Oni standing in the middle of a hall, challenging them to combat.  The hotheaded ones will probably charge in immediately.  Unfortunately, the Oni is actually floating just above the lid of a covered pit, and would-be attackers plunge ten feet onto punji sticks coated with toxic and paralyzing venoms.  (Or the pit could be filled with water and giant leeches.)  The Oni of course turns invisible and flies away laughing.

And so forth.  Think "fantasy Vietnam" and just go for it!

^
Thank you.

After the Oni tortures the players for a while, you could add insult to injury by having him assume the shape of a human "prisoner" (maybe a beautiful woman in a ripped dress) and beg the PCs to escort him to "safety" outside the dungeon.  It depends how gullible you think the players are.