Harry Potter being smaller than all first years (which we all knew anyway) but twice as feral.
Prefect Percy immediately noticing his small stature and being worried, Boy-Who-Lived or not. So he always offers him more help or just lends an ear. At first Harry is wary of him, no one is nice to him for no reason. Either it's a trick or with his new fame in the picture, because they want something but slowly he begins to trust Percy. Percy feels like a big brother for the first time in a long time. Ron and Ginn rarely went to him with their problems, they preferred Charlie or Bill (he couldn't blame them as he too preferred them but it still hurt), or worse the twins (that hurt worse). They didn't care for his advice. For his knowledge.
Harry was a breath of fresh air....well somewhat. Things he said were concerning and when Percy tried to get an adult to help (*coughMcGonagallcoughDumbledorecoughHisMomcoughHisDadcoughMADAMEPOMPFREYcough*) they brushed off his concerns which was more than a little concerning. So he just vowed to help Harry whenever and however he could. Then Harry somehow made the Quidditch team. At eleven. AT ELEVEN.
Oliver Wood loved his new seeker! He was timid at first but quickly found his confidence! Sure it was on the feral side but that's how Oliver liked his players! I mean, look at the twins! Plus Harry had the perfect seeker build! Though...he was on the small side. But Oliver chalked that up to the lid being literally eleven but when he looked closer he realized that no...Harry was really small. Too small. And Oliver would know, his mum was a healer and she taught him some stuff considering his love (obsession) with Quidditch.
So he pays more attention. And what he witnesses and notices is not...good. it's very telling. He tries to go to some adults about what he's noticed but nothing came of it and that really got him mad. Then out of no where Percy Weasley came up to him and started lecturing him on Potter's health and all Oliver could think was, oh thank Merlin I'm not the only one who's noticed!
He and Percy stayed up well past curfew talking about their smallest cub and somehow it ended up with them coparenting the Boy-Who-Lived. Oliver would be completely honest and admit he didn't think it would be so difficult until the troll incident....then the dragon incident.....then the CERBERUS incident!
Oliver became something of another big brother to Harry and another confidant. So when Harry told him more about what happened at the Dursley's (because Harry never called it home. Never) Oliver was already plotting to kidnap the boy from the house or the station whether Percy thought it was a good idea or not.