Oslo is small enough that you can cross half the city on a first date and large enough that the neighborhood you pick still matters. The mood at Sørenga at 7pm is nothing like the mood on Markveien at 7pm, and people read those signals whether they realize it or not. Here is how I think about the main contenders.
Grünerløkka for the easy first date
Løkka is the default for a reason. The walk along Markveien and Thorvald Meyers gate gives you a dozen options if the first place is too loud or too empty, and nobody looks twice if you bail on a wine bar after one glass and walk to the next one. Start at Tim Wendelboe on Grüners gate if it is daytime and you actually want to taste what the fuss is about. For evening, Territoriet on Markveien has something like 350 wines by the glass and a staff that will read your date better than you can. If things are going well, the riverside path along Akerselva from Ankerbrua up toward Mathallen is the move. Cross the bridges, look at the waterfalls, complain about the weather together.
The risk with Løkka is that it is the obvious answer, so half of Oslo's first dates are happening within two blocks of you. That is fine. It just means you should not pretend you discovered it.
Aker Brygge and Tjuvholmen for the view
If your date works in finance or has just moved here from somewhere flat, the harbor walk from Rådhusplassen out to Tjuvholmen does the heavy lifting for you. The Astrup Fearnley museum at the end of Tjuvholmen is open until 5pm most days, and even if you do not go in, the Renzo Piano building and the little sculpture park behind it are a respectable destination. For drinks, skip the chain bistros lining the boardwalk and go to Lille Herbern, the tiny ferry-accessed bar on its own islet, if it is summer. In winter, Vingen inside the Astrup Fearnley itself is quieter than it has any right to be.
This is a good neighborhood for first dates with people you do not know well yet, because the views give you something to talk about when conversation stalls.
Grønland if you want a real conversation
Grønland is where I send people who are tired of dating versions of themselves. Tøyen and Grønland blur together, and the stretch from Tøyen T-bane down to Grønlandsleiret has the best cheap food in the city plus enough bars that you can build a whole evening on one street. Postkontoret is a former post office turned dive that takes itself less seriously than anywhere in Frogner ever could. Asylet, in an old wooden building from the 1730s, has a courtyard that doubles your usable date hours in summer.
For dinner before drinks, Pakistani food at Punjab Tandoori on Grønlandsleiret is around 150 to 200 kroner a plate and is the test I use for whether a date can handle a place without tablecloths.
Frogner if the date is already going somewhere
Frogner is not a first-date neighborhood for most people. It is what you graduate to on date three or four, when you want to walk through Vigelandsparken at dusk and have it feel like a decision rather than a tourist itinerary. That said, if you both work near Solli plass or live on the west side, the strip around Bygdøy allé has a few solid options. Lofoten Fiskerestaurant is over budget for a first date but underrated for a fourth. Palace Grill on Solligata is famously walk-in only and has a good bar to wait at, which is itself a kind of first-date activity.
St. Hanshaugen for the underrated middle ground
If Løkka feels too obvious and Frogner feels too much, St. Hanshaugen splits the difference. The park itself is the move on a clear evening. You can pick up a bottle from Vinmonopolet on Ullevålsveien before 6pm (or 3pm Saturday, plan accordingly) and walk up to the top for the view back over downtown. Kaffebrenneriet on Waldemar Thranes gate is fine for daytime. In the evening, Bar Boca is the size of a living room and forces you to sit close to your date, which is either a feature or a bug depending on how the night is going.
How to actually choose
Match the neighborhood to what you want the date to feel like. Løkka if you want options. Aker Brygge if you want to be impressed by the city together. Grønland if you want to find out who they are when the lighting is bad. Frogner if you already know. St. Hanshaugen if you cannot decide. Whichever you pick, do not plan more than the first stop. Oslo rewards walking, and the second bar should be a decision you make together.