Sukûru ranburu OVA: Ichigakki hoshû

If you haven't seen the original series of School Rumble yet you should watch it before watching this OVA or it will be pretty hard to follow as it assumes you know all of the characters already. The stories aren't that much different from the episodes in the series for the most part although the flash back showing the classes prehistoric ancestors hunting a mammoth was somewhat surreal as the characters behaved in the same way as their modern day counterparts.

Sometimes OVAs use the fact that they aren't constrained by television restrictions to include large amounts of fan service; although the class do get naked on a trip to the hot springs nothing below the girls' shoulders is above the water, the only character to be seen naked (from behind) is Harima. That scene is pretty funny though.

If you enjoyed the series I'd recommend seeing this too, the subtitled version can be seen legally on Funimation's YouTube Channel.