Hi Hotrod:
Looks like you got this one identified. It certainly is NOT a great tree, if you value local native plants and environments and don't want invasive exotic pest plant seedlings coming up everywhere.
HOWEVER: it is definitely one of the largest Buckthorns I've ever seen. You may want to have its identity confirmed by a local arborist or native plant enthusiast group - and have them document this individual for future promotion of what happens when one doesn't pay attention to a stray seedling. "LOOK HOW BIG IT CAN GET!!!"
That location sure looks like a very common situation that I have observed innumerable times over my career. A seedling starts growing in/next to a purposefully planted tree or shrub. Gradually, either the purposeful plant dies or just gives way to the invader - and finally only the invader is there, usurping the landscape space of something far more preferable.
As I mentioned in your other post, you might visit the Toledo Botanical Gardens for good ID help if other human resources are not available.
I suspect you have a very interesting landscape to walk around in. If you had been posting these things early last fall, I'd have arranged a drive-by as I took my vacation jaunt through OH-MI-WI. You would have been right on my flight plan between Cuyahoga Valley National Park and Hidden Lake Gardens...
