My first thought on the title was it was some kind of twist on UFO (Touhou's 12th mainline game). Then I listened to the song and I was intrigued. Then I heard the whole song while playing and it's actually really nice and whimsical. It wasn't an immediate favorite like A Turtle's Heart, but I do like it.
Osu! is a rhythm game based off of Osu! Tatakae! Ouendan! and the American release Elite Beat Agents. In these games you click circles to the rhythm of the song. Osu! also has different rankings you can shoot for, with SS being the best (Hit every note and have 100% accuracy). THere are also mods like Hard Rock, Hidden, Flashlight, and Double Time/Nightcore if you want to increase the difficulty even more.
There are 4,503 views in 935 videos for osu!. His channel published over 1 day worth of content for the game, roughly 21.26% of osu! content that JonathanScarlet has uploaded to YouTube.