I don't know about the OMS-7 but I assume it was a newer or higher end model of the OMS-5, which I have. The sound quality of the OMS-5 is excellent, even by modern standards, it is a reference quality player.
The OMS-5 retailed for $1k back in 1983 so factoring in inflation, if that were today's market it would have retailed for $1897, which puts it in the mid-fi reference cd player market. Like I said it is a very detailed cd player, and sounds great, the only problem it has, and I don't know if this was solved with the OMS-7 or not, but the laser has a tendency of just dieing for no apprent reason. When it was in father's possession he had the $500 pick-up replaced twice, in the time it was mine I had the pick-up swapped once from an OMS-5 with a crapped up chassis, and it has died again recently.
If Nakamichi fixed the laser problem with the OMS-7 and the sound quality is just as good, I'd say grab it. If the laser still breaks, don't. Nakamichi was bought out in the early '90s and parts are now impossible to find.