Oasis does not have a Viking Lounge.
The Crown Lounge for Diamond, Diamond Plus and Pinnacle members is a nice venue on deck 4 convenient to the theater, ice rink comedy club and Casino. The room is nicely appointed with comfortable seating and offers continental breakfast as well as evening appetizers. Sometime cookies/fruit during the day
It is open all day but has specific hours for when the concierge is there. During evening cocktail hours, if you want a drink, you will use one of your pre-loaded vouchers or pay with the sea pass card.
Recently some cruises had many crown and anchor members expected to use the Crown lounge some hours might be restricted for Diamond members. If this happens, it is generally during the evening cocktail hours.
Since the drinks are complimentary., many tip the server $1 or $2 per drink. If the concierge has help you during the cruise (i.e. several reservations) it is customary to provide a tip to him/her as well.
Enjoy