Oliver here as a yandere would be caring but not to mention incredibly possessive, clingy and EXTREMELY competitive and obsessive of you as well. He's actually humble about his talents and stuff but when you're around, he can't help but show off a bit for you since he does need to impress you after all. He hopes you fall in love with him after watching him play Quidditch. And he's as serious as making you only his no matter what, just like how he's serious about Quidditch

You could be from a different house and be on the Quidditch team of another house but he just sees this as a way to prove himself to you that he's worthy of your love and affection, he's driven by you to win. It's like you're his source and ray of inspiration, a pillar of strength for him, always pushing him to do better and do his best. Of course, when he shows off too much around you the other members of the Gryffindor Quidditch team are ready to call him out on his behavior much to the amusement of everyone else there and to the embarrassment of Oliver. "Oh, Oliver, is that Y/N I see?" asked Fred in a sing song teasing voice and Oliver tried to quickly look around to where you were and when you flashed him a smile from the stands, he could swear on Merlin that his heart did a somersault

The others started snickering after seeing Oliver's furious blush and who did he think he was fooling? No one was buying the 'I barely even noticed them' nonsense. "They look good don't they?" teased George and Oliver replied in a dreamy way with a small smile on his face "They look good every day... wait... what? HEY! Focus on the match, we need to beat Slytherin today at any cost no matter what!" he turned firm and strict again, making sure his team mates trained hard to beat Slytherin no matter what. His hatred for Slytherin didn't just stop at Quidditch, it just became more stronger when he saw Marcus Flint, the Slytherin's Quidditch team captain talking with you about something

He didn't know why as soon as he saw that sight he felt like either punching him in the face and sending him to the hospital wing or hexing him, a hex involving something about him not being able to use his mouth or tongue anymore to talk with you. He didn't know why he felt so bad, bitter and a mix and storm of emotions flurry inside him. Ranging from rage, anger to resentment and fear. The fear of losing you, for someone like him. No, he would not allow it. He would NEVER allow it, never in a million years, He can even lose in Quidditch, there's always a next time to win but he just can't afford to lose you under any circumstances

"Is he bothering you Y/N?' asked Oliver as he suddenly rushed to your side and distanced you from Flint. Flint scowled and you smiled and replied "No Oliver it's all right, he was just asking me about the homework we received from Potions by Professor Snape. I do have to admit, writing a 5000 word essay based on Potions all around the world and it's effects is quite difficult'' and Oliver immediately needed to get you away from Flint. He didn't like you spending time with him and besides, he could deal with him later on. "Would you like to watch me practice if it's not too much of a hassle? I really look forward to seeing you there'' and you smiled and told him you'd come with him saying bye to Flint

It took him all his restraint not to punch Flint for flirting with you after the practice was done. He wasn't mad at you, of course not. He could never be mad at you, in his eyes you're literal perfection, the definition of a god/ goddess who could do no wrong. Even if you do end up doing something wrong at some point in your life, he will find a hundred different to justify your actions. He'll do his best to keep other guys away from you. Will he stalk you? Well, he prefers to call it 'looking out for you' after all, not everyone at Hogwarts is to be trusted like the Slytherin Quidditch team or Roger Davies for that matter, the irritating hindrance between you both

Oliver is also very protective and a fussy yandere when it comes to you. He treats even the slightest of the scratches like a flesh wound by someone who stabbed you or something. If at all you do get hurt during Quidditch, he'll make you rest, he doesn't want you playing or doing anything when you're injured. He'll drop everything and rush to you, after making sure you're all right he'll carry you bridal style to the Hospital wing for a checkup by Madam Pomfrey just to make sure you aren't really injured. However if someone deliberately injures you, he isn't going to go easy on them. Not only will be make them suffer the humiliation of losing to him and his team at Quidditch, but he'll also break a few bones and send them packing to the Hospital wing, and a few hexes and jinxes won't really go amiss either

If you keep getting injured or flirted with by people, he'll grow anxious for your safety and after feeling slightly insecure, he'll have to kidnap you to save you from all those scumbags. It's for your own good darling, just trust him he'll take care of everything for you