How prestigious are the meetings of the Optical Society of America (OSA)?
AFAIK for disciplines related to the Optical Society of America (which is a well-recognized organization), the most prestigious things are journals - either by it (for example, JOSA A, JOSA B, Optics Express) or not (e.g. general physics or general science journals). When it comes to conference, its importance is related only to speakers/participants (so there is no general rule).

In any case, a poster presentation is never prestigious, be it by OSA or not (usually almost everything gets accepted).

I would ask that not every field have well-defined tiers for conferences. For example in mine, there isn't. And if you care for prestige - if you can't tell the difference - the chances are people seeing you CV won't be able to tell the difference either.

Usually review of abstracts (especially if no feedback is given) is to decide which things are suitable for talks (the best), posters (almost all others), and which - neither (they make no sense, are on another topic, of very poor quality or are OK but there is limited space for posters for some reason).