From Wikipedia (look up "socialist mode of production"): "In Marxist theory, socialism (also called the socialist mode of production) refers to a specific historical phase of economic development and its corresponding set of social relations that supersede capitalism in the schema of historical materialism."

While Marx viewed "socialism" as a transitional stage between Capitalism and Communism, and he was particularly eager for every economy to eventually become communistic, you can't say that he wasn't also an "advocate" for Socialism. For him, it was an essential first step in the inevitable development towards Communism.

As for AOC, she may not SAY she's for "socialism," but her policy preferences are clearly socialistic, in any historically relevant sense of the word.