Ao Haru Ride OVA should be definitely watched right after season 1, but the most important part of it is ep. 2. From my experience, the first season ended with a cliffhanger and of course as a viewer I had a lot of questions, which got answered here. Nevertheless, they give u another cliffhanger right in the end (that's why you should watch the last episode till the very end and pay attention to what they say). Atm (2021) there is no season 2 and I don't think there will ever be, so the best option would be to pick up the manga and start fromthe chapter that they finished on.