Presently the US has oil sanctions on Iran, Venezuela and Russia. These are on this short list precisely because they are dictatorships with bad human rights records.
Meaningless shell game. It's like someone not wanting to spend money on groceries, and agreeing to pay the mortgage instead as long as their partner pays for the groceries.
The same groceries are still bought. The same money is still spent. It even comes out of the same accounts but is just split up differently. The net spending is still exactly the same.
Europe buys someone else's oil and the US buys the oil they might have otherwise. It's probably all handled at the spreadsheet level by notionally changing who owns what.
But it's a decent con for the easily fooled to make them feel better that they are "doing something", just like campaigning for human rights while purchasing products that perpetuate slavery and misery.