Thanks for the advice LovingRadiance.
While so simple, it wouldn't have dawned on me to not bring up relationship issues too early, but it makes perfect sense.
Anyway, me and "A" had a heated discussion about things the other day. Mostly about how she would have jealousy issues, and how she's still concerned that I haven't known "O" long enough. But I'm still optimistic, since she is still entertaining the idea of trying a poly relation, although skeptical. I guess if things will work, I'll have to take things slowly with her, which is fine by me cause I'm thinking long-term anyway. I still plan to bring up the idea with "O" soon, just to gauge her reaction. But "O" always seems to surprise me with her view on women, such as how some girl looks sexy, or how she wonders what sex with another woman would be like. So "O" seems like the safe bet so far, but the catch is that "A" would make the balance work right, and is not really replaceable. I'm fairly certain they would get along too, if given a proper chance. For now, more things to learn about.