When they don’t have work to do, they travel. Percy once took Oliver to visit Charlie in Romania. 

“Can you imagine flying over these mountains, Perce?”
“Don’t even try it.  I know you.”
“I won’t get eaten!”
“You do know they shoot fire..?  Charlie don’t encourage him!”

Fred and George found out about Percy and Oliver because they found a Quidditch sweatshirt in his drawer and “No one in our family would ever like that team.” Ron found out from Harry who saw them snogging after Quidditch practice. It went something like “When did Percy and Oliver start uh you know.” Ron turned almost purple and his freckles nearly shot off. 

Percy hates all those “wood” and “broomstick” and “ball” jokes. But it’s kind of cute if Oliver’s the one making them. The eye-rolling kind of cute. 

Percy has never, nor will he ever, called Oliver Oli. 

When Oliver moved in with Percy he couldn’t understand how one needs so many books. Ditto Percy and Oliver’s brooms.