Yep. Buggy or wagon axle. Possibly very early car of some kind. Who knows why it was drove in ground maybe to tie out horses or mules so they could graze.
If you need it gone dig down and cut it off or if you really want to pull it dig down a couple feet or so beside it and see if it Will move probably has a bearing race on other end also.
Old axle for something small, probably horse drawn.
The square to round transition at a forged in bearing race is typical of them.
The threads are for the nut that held the hub and outboard bearing on the wheel.
Bet if you look closely it has rusted to almost a wood grain pattern.
Thats typical of the early iron axles. They have a lot of silicon inclusions that get drawn out into lines during manufacturing process and don't rust at the same rate as the surrounding iron. The iron is very soft and prized by blacksmiths because it forges more easily than the modern stuff. Pattern welded knife makers also want it because it forge-welds easily.
When it was driven in, it was probably a just another piece of scrap metal from behind the barn.
Any chance it marks the location of an underground water pipe or something similar?
Likely an old oil well or possible water well. I would call OREB to have them come have a look at it. If old oil well, they need to plug it to keep seepage from affecting water table. It should cost you nothing to have them look at it.