Oliver doesn't care who knows but Marcus does. So Oliver gets great fun out of teasing him, shooting him looks when they have a class together, almost coming up to him in the hallway.
Once he dropped a flower in Marcus' lap in the Great Hall. "Sorry," he said. "I was bringing that to McGonagall." Marcus crushed the flower and said nothing.
Oliver always pays for this later. That's why he does it.
FINIS
....
unfortuantly, unusually, i have nothing to say.
EEP!