Since Buffett is so heavily invested, don’t you think he would help restructure their debt if it came to that?

Why? Do you think this would be in the best interest of BRK shareholders?

The OXY holdings in the Permian would have value in a bankruptcy. And BRK would be pretty well positioned to recover debt, at least in part. And they've received good interest payments for some time already with some principle.

Basically is OXY a good long term stock investment? Or could the significant losses be useful as a tax offset?

Frankly, I can't tell if Buffett is emotionally attached to OXY via his admiration for their CEO or not. I don't think he understands the O&G business in depth. He's basically said so. And he's reversed judgment decisions in the past - e.g. IBM.

I'll trust whatever judgment Buffett makes on this subject. It's pretty trivial to BRK in any case.