The OVA is based (only small part of it) on a very popular manga from Ribon by the same title. The story focuses on relationship between Nao and Uehara. Nao lives far away from school, so she needs a nearby apartment from school. Lucky for Nao, she can live in an apartment with Uehara, a really popular high school boy.
But since they are going to the same high school and people will easily criticize that a high school boy and girl are living together, therefore Nao and Uehara tried to hide the fact that they live together in an apartment. It's actually not hard if they don't have romatic feelings toward each other, not to mention there are other people who have crush on either Nao or Uehara...