urm prob has easy answer but how do u tell when ur about to run out of air when under water.

Once you start losing health.

Losing health, gurgling?

that it didnt oblivion have a 02 bar or sumit

Yes, but it's not in Skyrim.

As far as I know, there's no meter that tells you how long you have... you just start getting hurt eventually. Controller vibrates, you grunt & gurgle, health bar drops.

You can prevent that by either finding a charmed necklace, helmet, etc. or being an Argonian

Yea, in Skyrim you know when you start taking damage. Be nice if there was some sort of bar timer as was in previous games. But it is simple enough to keep a necklace or something in inventory. Another thing is once you get enchanting high enough that you can double enchant your hats, you can just throw this on your hat-along with the archery enchant(or whatever) and then never have to worry about it.
One curious thing is that even when wearing the water breathing item, you still gasp for air! You just don't take the damage.