original equipment- means exact same part made to same spec as the original.
original equipment manufacturer- means the manufacturer who made the original part.

Simple, but of course there can be play on words BS such as an OEM supplying a part but not the part that is being talked about, and sometimes the genuine or aftermarket part can be an improvement on original equipment.

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1988 325is (purchased new) sold
2004 X5 3.0 2005 X3 2.5
2008 X5 3.0 (new to me)