Hello and Welcome. Another Big Island diver joins the forum; cool man. A 120 Rob Allen is a big gun. Big enough for Mahi and it will handle an Ono as long as you are rigged properly. Personally I would opt for a bigger gun. I used to have only one gun a 55" Wong, 9/32 tahitian shaft, powered by 3 5/8 mean greens. This gun is deadly accurate but lacks enough range for the Ono. I have hit a few with this gun but have not landed any to date with it. My bigger gun shoots 5 bands, 5/16 shaft and an Alexander slip tip. This has more than enough power. The Rob Allen should have enough range but you should definitely go with a slip tip, breakaway set up. As far as flotation goes, do not opt for an inflatable float. Some may beg to differ but I have seen and heard from other divers hunting Ono that these inflatables are not enough. As soon as they are taken under, they start to collapse. Since you are in Kona, visit Rob at Blue Water Hunter and pick up two 11 liter Rob Allen foam floats. You don't need to worry about collapse with these guys. A standard vinyl float line with parachute chord inside will be do okay but if you got the dough, pick up a bungee. You can go the expensive route of 75 foot or the cheap route of 25 foot attached to a vinyl float line. Again, go see Rob and he will be able to get you what you need. Actually talking to him would be a good idea. The guys is a really good reef and blue water diver. If you got more questions PM me with your phone number and we can discuss more detail; maybe even go shoot some pelagics together.
Brad :martial