Probably not, although I have no idea what you're seeing in fields there since I'm at the opposite end of the state. We do have several native forbs that look similar to Giant Hogweed but remain much smaller (it gets huge).

The U.S. Distribution Maps for Giant Hogweed do not show it in Oklahoma yet.

I'll link a distribution map below. There are several that can be found by googling and none of the ones I looked at showed it in OK.

Dawn

Giant Hogweed USA Distribution Map

You might be seeing cow parsley which looks similar and can get big under the right conditions. Can google 'cow parsley vs hog weed'.

Or hemlock.

ok thanks