The Tokyo American football team, the Seagulls, play regular games at the famous Tokyo Dome. Always accompanying the team is the lively and cute cheersquad. Cheer leading is actually quite popular among young girls in Japan, and despite the fact that cheer is something that kind of developed from an America sport, cheer takes many forms in modern day Japan; they even have cheerleaders at baseball games (shown in the picture)
The "O-en dan" is something you'll see at sporting events or sport meets, and is basically just a group of people who lead chants and cheers. In a way, the modern day cheer groups seem to be extensions of this, with of course a sexed-up image to fit in with modern times and entertainment.
Being part of a cheer group or "o-en dan" is seen as participating directly in the sport. Japanese cheerleaders tend to be more on the "normal girl" side, as opposed to the exclusive look of say American cheerleaders, and for baseball for example, it's just more for fun.