That's the most common usage, yes.
They're not proper English, however they are used (by preference) when it makes a sentence more semantically readable.
Proper:
The guys I hate are Monny, Mindy and Barrie.
Oxford:
The guys I hate are Monny, Mindy, and Barrie.
The Oxford reads better. The Proper reads like two groups (Monny) and (Mindy and Barrie) rather than the intended three.
And, as you have rightly noticed, Oxford commas crop up in natural sentences too, despite it not being Proper English.
Has anybody noticed that spellcheckers in some apps and applications are actually pushing Oxford commas? It's a ****in conspiracy