Acording to this answer, at low stack NLH tables a 25-buy-in is enought, while 40 or 50-buy-in is needed for middle and high stack tables.

This don't differs too much from what Jeff Hwang estimate for PLO:

I think a 30-buy-in bankroll is probably adecuated for a small-stackes live game.

and

[For higher-stackes-games]. For these types of games, you are probably goign to want closer to 40 or 50-buy-ins (if not more).

So there is not a big difference between the bankroll needed for NLH and PLO.

Jeff Hwang: "Pot-limit Omaha Poker: The big play strategy"

Acording to Jeff Hwang the bankroll needed for PLO is a bit bigger than for NLH:

While pot-limit Omaha can be an exceptionally profitable game, the fact that you will probably need a bigger bankroll to suppor playing PLO vs NL hold'em is in itself a cost.

Jeff Hwang: "Pot-limit Omaha Poker: The big play strategy"