My own exp. is from pre-covid and then one had access all the times-without any need even to show proof you would go by train. Always nice and welcomed. As  in Austria there is no need to must have RES you can always imagine a train you would take-they will not allow you in to sit for hours. I am not sure if this has changed, but I think not.

Also: several classes of bookings on the NightJet-not just 1st-are also entitled to lounge access before that train departs

BTW-<OeBB also has staff-bistroś where anyone can also go in and eat-self serve-higher prices for non-railway workers, but for Austria still fairly cheap. Always near a main station, but in general only german spoken

Hi @Giuseppe Iannuzzelli 

Indeed you can use the OBB Lounge with the 1st class Interrail/Eurail Pass. However, there are some conditions: The booked connection must be possible with RJ, ICE, EC, IC or D trains.

Please follow this link for more information: https://www.oebb.at/dam/jcr:82caaf71-79de-42ad-be01-5a6fb008d537/folder-lounges-benutzungsbestimmungen-en.pdf

I wish you a lovely day!