The only thing I can find that is really 1/8 thick is Masonite (or some similar hardboard-like material)
The Subfloor is 3/4 OSB, in good shape, but I want to match the level of another area.
Is the addition of 1/8 Masonite layer kosher ?
(nailing 5/8 bamboo with 2" staples)

I can't think of a compelling reason to NOT use masonite. The layer is thin, so it won't significantly affect nail-holding.

Lacking any objections, I am forging ahead.

I've used it to bring up a floor so the wood flooring matched the height of a tile floor. I think you're correct in that that 1/8" shouldn't effect the 2" fasteners ability to hold into the subfloor.

How would you adhere the masonite? Glue or staple? Does it make a difference?

How about 1/8 luan? Easy to work with and you can glue or staple it down.

Thanks for the replies.

I actually did bring home some "1/8" Luan, but it really measured closer to 3/16. I am trying to match the level of a preexisting oak floor, so I wasn't happy with that difference. Maybe this is too picky, but I just got my mind set on very equal levels.

I am planning to use a minimal number of cement-coated 8d flooring nails to hold the masonite down, but then use the 2" staples every 8"-10" through the bamboo, so I think it will all be well attached in the end.

I mark out the joist positions so I can hit joists wherever possible. This is at least 2 times on every floor board.

Thanks for the feedback !

You could loose lay it and let the flooring fasteners hold it down.