Writing isn't terrible because you don't pay attention.>Now suddenly she is crushing on Marco and has no attraction to Oscar.>There isn't even any explanation as to why Star suddenly doesn't like Oscar anymore.
Star clearly finds him hot in BMB when she wasn't thinking of him as her best friend & housemate, accidentally called him Oskar in Royal Pain (which means the crush is roughly as strong at this point) and is very heavily implied to have a crush on Marco in S2E1, while she still had one on Oskar a third of the way into season 2 (Sleepover, just second at that point). Star's attraction to Oskar faded away after he dropped out because she forgot about him.>because if she liked Marco she would get jealous like she did in Bon Bon the Birthday Clown and not go out of her way to make sure they get together
Star found out about his crush on Jackie in S1E2a, before she would logically feel any jealously, so she was already fine with it by some point.>It wasn't until after Marco and Jackie became a thing that she knew she couldn't take Marco away from Jackie
But she already must have "knew" that by Freeze Day, learning that Marco's had a crush on her for an entire decade.