I think Billy is trying to teach Oscar to hunt.
I shut Oscar in his suite to give Billy some private playtime. Once Billy was done, I let Oscar go after the pom-pom-on-string toy Billy had been chasing. He was perfectly happy to chase it around on the floor, and onto low objects like the library's futon. But if I tossed it up a cat tower he just sat on the floor twitching his tail.
While Oscar was wondering what to do, Billy walked up, climbed the tower, retrieved the toy, and dropped it at Oscar's feet. All very casually, with nothing like his usual energy. After this happened a couple of times, I decided Oscar's problem might be that I was tossing it up too high. (He can climb just fine, but his sight lines are lower.) So I flipped the pom-pom onto a lower shelf, and then a lower one. Billy sat and watched, Oscar sat and watched. Finally, Oscar looked at Billy, looked at the toy (on the lowest shelf, now), and pounced.
No idea what Billy thought of all this, but I told him he was being an excellent big brother and gave him extra kitty treats.