[MENTION=101176]browniies[/MENTION], Rooting simply refers to the process of granting yourself superuser access to your phone. In computer terms, that would be like giving you administrator access to your PC. Since your phone is already rooted, there's no need to root it again. What you want, based on your question, is to put an ICS or JB rom on your phone (a "ROM", in this case, is analogous to an OS distribution).
Anyway, CM10.1 (JB) stable has just been released for the Optimus One, but I personally wouldn't recommend it if you're a heavy user of wifi or 3g (as I am) since the battery drains pretty quickly. There's also the issue of the system partition being a little bit too small, and though their are workarounds, it's a rather more involved process. If you want a newer Android OS on your phone, I would recommend
CM9.2.2 (ICS) from hephappy. It's super stable and bug free. Just download the ROM, put it your sdcard and flash it via recovery. If you don't know what I'm talking about, get the guy who rooted the phone to help, or contact me via PM either here or on XDA.
Good luck!