PSO Series are the original optic for the SVD Dragunov rifle. The military versions are PSO-1 and the more modern PSO-1M2 briefly seen on civilian Tiger rifles that made it into the USA in about 1994. PSO's are manufactured by NPZ in Novosibirsk Russia, the oldest optics manufacturing plant in the country (since about 1904).
For much more detailed information on the PSO, visit Dragunov.net here

Examples of PSO types:

Top: Rare PSO-1 made by Izhmash (Not NPZ)
Middle: Chinese JJJ 4x24 (English marked for NDM86 rifle)
Bottom: BelOMO POSP 4x24. Note the MTK83 AK mount instead of the two SVD mounts

Izhmash PSO-1 with IR detector

Auzie with NPZ PSO-1M2, which is the modern version of the PSO-1 seen here on an Izhmash Tiger rifle

NPZ PSO-1 on NDM86 rifle





PSO on RPK. This is possible because the RPK has an SVD style rail with center groove that allows a standard PSO to slide all the way forward. This is not possible on a regular AK rail because the stop pin at the front of the PSO mount will not let it seat on the rail properly



And here is an example on a PKM



Here are two examples of what are probably POSP's in the Ukrainian-Russian conflict on standard AK rails. While I can't see the other side of the optic they are most likely using MTK-83 mounts