Assuming it is an amplified show, in a small room, the PA will probably be reinforcing that which isn't coming from the stage, so mainly amplifying the vocals and keyboards say. The omni in the audience should work, though at that point, you'll be getting the whole mix including the onstage sound and the PA reinforcement, so you will have to keep this in mind in mixing the two sources in post.
It might be better to run the omni onstage or close to stage lip, as long as you are sure you can get what won't be reinforced to much by the PAs -- namely the drums, guitar amps, and bass amp (though admittedly, capturing these all with the right balance between instruments with only a single omni might be difficult).