I whacked off a couple or more larger oak roots here about this time of year trenching in a sprinkler system.

I was worried about one major sized root about 6 foot from one tree, it appeared to not bother it, but a neighbors tree diagonally about 20 foot away had a major branch die not long after. Not sure if I caused that or not.

You would be wise to have a certified arborist out to look at the site and suggest a "best" method.

Cutting the roots that close to an your oak will seriously damage it, mostly likely kill it.

But it may be possible to tunnel under the roots rather than cut through them. It may be a bit pricey -- you need to weigh that cost that against the cost of losing/removing an established tree.

Thanks. I do have a lot of trees... it's just that this one sits somewhat alone in a more "open" area of my yard.... I guess I could just have it removed.
It will probably cost $1,200 to remove it. Not sure what boring underneath would cost. Problem is, if I bore underneath, there is no gaurantee that it will save the tree.

True but you will likely spend less on an opinion from an arborist and might end up saving money in th elong run versus removing a dead tree.