The OVA episodes come from content in the manga that was not animated in the main TV series. It feels like the typical anime episodes, except that some episodes have a more serious theme, one of them involving violence and abduction. The animation quality is superior to the main series.
Regarding watch order, some watch the entire OVA after the main TV series. But some watch orders estimate where exactly in the TV series you should watch a particular episode, but watch out for OVA episode 8.
I watched it after movie 2. Its story is after episode 48 but before the first movie. So it's a bit jarring because I was watching an earlier point of the series, whereas the two movies have moved way past the TV series in terms of maturity.
There must be two reasons why some watch orders place this last. 1) Just in case you're disappointed with the second movie. I enjoyed movie 2, but not everyone does. It's a way of leaving one good episode as your last exposure to the Kimagure Orange Road anime. 2) minor spoiler: It's only in this episode that Ayukawa counted the steps on that fateful stairway where Kyouseke met her first. It's only here does she realise that Kyousuke's count of 100 is correct.
That said, there's nothing in this episode that negates your enjoyment of the movies if you watch this first.
Anyway, treat these OVA episodes as additional episodes to the main TV series. You can't go wrong watching them.
