Kimagure Orange Road OVA is basically the extension from the TV series. The time frame is probably in the middle of the TV series.
Order to view with the TV series.
White Lovers - among the episodes 35 and 45
Hawai Suspence - among the episodes 17 and 25
I was a cat, I was a fish - among the episodes 17 and 30
Hurricane Akane, the shape-shifting girl - among the episodes 20 and 25
Spring is for Idols - among the episodes 23 and 30
Birth of a Star - Right after "Spring is for Idols"
Unexpected Situation - after the OAV of Akane
Message in Rouge - after the episode 48
Thanks to the success of the TV series, Izumi Matsumoto had decided to continue the Manga which might be why this OVA series was released afterwards. Unfortunately, it's not a direct continuation of the TV series. Well... it kind of is and isn't because it rewinds back two episodes before the end of the TV series and does a bit of re-shuffling with chapters of the Manga that never made it in. Characters are now in senior high school instead of junior. The first half of the series are mostly filler episodes and characters such as Kyousuke's cousin Akane makes her first appearance. For the
most part, it's fan service with the girls. Then the second half gets a bit more serious and is mainly based on chapters after the TV series.
If you want a prelude before watching the spin-off movies or just miss the trio after finishing the TV series, you'll probably like this.