Ophthalmosaurus icenicus lived in the Jurassic seas that covered southern England 164-150 million years ago. The rocks in which the fossils are found were laid down in warm shallow conditions. However, since there is a lot of evidence that Ophthalmosaurus was a deep diver it probably spent much of its time in deeper waters. Many other marine reptiles shared the seas with Ophthalmosaurus. The Oxford Clay deposits have produced many species of plesiosaurs (e.g. http://www.nhm.ac.uk/nature-online/species-of-the-day/peloneustes-philarchus/index.html) and marine crocodiles, a wide variety of fishes including sharks, holocephalans and bony fishes as well as belemnites and ammonites.