Just rub in a bit of mineral oil from time to time and you'll be fine - wipe off any excess oil when you finish rubbing. Since it's new, I would rub in a little oil every few days, just to be sure -- after a couple of months, you can just do it whenever you think about it. It helps ivory a lot if you carry it -- it works best if it gets moisture and hand oils from carry and use (and will look a lot more beautiful). Use pure, food grade mineral oil (you can get it from a pharmacy -- do NOT use any mineral oil with fragrance (like baby oil) as it will stink forever). You can also use orange oil (Formby's for example) or lemon oil (usually available at music shops for oiling woodwinds I think) -- just make sure you don't use any oil that will go rancid with time (don't use olive oil, or most vegetable oils, for example, since most go bad with time).
I wouldn't really advise dunking the knife in mineral oil or soaking it for any period -- it can sometimes soak in and discolor ivory.
