If you're talking about submerged PC's, I would strongly advise against it. Its not really that effective (or necessary for that matter), it voids the warranty on every single component, and they're difficult to maintain, cumbersome to relocate (its a friggin fishtank for crying out loud) and a nasty mess to work on if you have to swap a part or troubleshoot.
I guess the trade off, is,, it does look pretty cool, but its not something I would do with high dollar parts.
But the key is, water cooling uses a liquid to dissipate the heat more quickly. Oil cooling is doing that, but rather than blowing off the heat through a radiator it is simply using the sheer mass of the amount of oil to offset the heat and allow it to rise naturally.
Once again the only difference is that it isn't a sealed system. The advantage of it being that every single component gains benefit from it