I'm not sure whether you support Starco or not, I know I do, but I had the same concern you bring up. I asked Dominic Bisignano, the supervising producer for the show, if Star ending up with Oskar was possible had Toffee not shown up. The way I see it with his answer was yes, if Star spent more time with Oskar (like during the summer as Marco spent more time with Jackie) then yes she could fall for him again despite her feeling things for Marco.
But ultimately, it's just ONE BIG """"IF""""".
It didn't happen, Toffee showed up, Star decided to confess and that's the way the story is moving forward.
But the reason why I asked this in the first place is because while I ship Starco a lot, I was in Oskar's shoes/position once in HS. The girl I spent time with only did it to get her mind off the close guy friend she had a crush on, and only because he got another girlfriend. I eventually got with the girl and we spent a couple of years together before we decided it didn't work out. But I found out through her friends, and eventually a confirmation from her, that she didn't like me at first for various reasons but spent time with me to get her mind off her crush (which was her friend). And she ended up liking me a lot during those 2 years after giving me a chance. But to be frank, despite the nice times I spent with her, I didn't like hearing that. I felt used. Just my worthless 2 cents.