I've used the lounge flying to SVO. All Skyteam airlines use this lounge, it's after security but before passport control, so it can be used by non-Schengen airlines as well.
Think a smaller/cheaper version of the CSA (non-Schengen) Crystal lounge at PRG minus the shower and computers (there are some computers but they only have access to a limited amount of websites related to airlines and/or OSL). Not bad but rather unremarkable.
AFAIK it's not a BA or SK lounge. It's a generic airport lounge used by all the ST airlines, plus BA and 1-2 others (AY?).