who is the oldest?

In LOTR two beings are claimed to be teh oldest, Tom Bombadil and Fangorn. It is said that Tom Bombadil is eldest but later Gandalf tells someone that they are about to meet the oldest living thing, (in refrence to Treebeard). Is this because maybe Bombadil is immortal and Treebeard is not? What the hell is Bombadil?

What Tom is is intentionally (on Tolkien's part) a mystery, so noone really knows for sure.

I've thought about that Treebeard/Iarwain business too. Tom is certainly long lived, and has certainly been around longer than Treebeard. He was there for the first rain drop and first acorn, while there are trees older than Fangorn (according to Fangorn).

The answer, I think, is that Tom must not be considered (at least by Gandalf) a living thing, but a spirit. He's never stated to be specifically alive, but he's certainly a spirit.

Also Maiar and trees must not be included in the 'oldest living thing' statement, for the Wizards were there before the world, and Fangorn notes there are trees older than he.

i always liked to think that when the valar were singing with illuvitar creating arda that bombadil was the embodiment of illuvitars own private song, such as the strife was created by melkor

I think you strike nearer the mark than many.