To me it looks like carpenter ants have taken up residence. Or else it's just the result of the trunk having been damaged in the past. I will say that it looks like your oak tree has some pretty harsh living conditions.

Is it just the trunk you are concerned about? Is the tree budding?

It's not Sudden Oak Death. That's a rapidly fatal disease that usually has a different appearance. The tree has some sort of canker that's bleeding. Given the size of the cavity in the tree, it has significant rot issues. I agree that it may have ant problems. It's an old tree especially for its living conditions. These things happen. It's hard to tell the extent of the damage from the picture. Usually if the tree is in trouble, large branches in the canopy will start dying and falling off in windstorms. At that point, the tree should be cut down if it's near a house or something of value that could be damaged or destroyed.

The house was built in the 1890s and almost all of the trees on the property have been taken down already. This is one of very few left, I'm not sure if it's original or not. It's in full bud right now, I was definitely checking that out. So it looks like, at least for now, it's getting nutrients all over. It's back far enough that it wouldn't hit the house if it came down, but it would definitely pull down some power lines. I just don't know when it becomes a "bother the property owner" situation - I guess when branches start coming down?