This is up the road a bit from Opera, by St Hanshaugen. Mostly it has a very watery theme, but Tuesday and Thursday nights (except in the imddle of summer) they also get a couple of Opera singers to come in. They sing four sets of three songs usually, with a break of about 20-30 minutes between sets. Although the sets are usually introduced in norwegian, a request to the publican and impresario might get him to speak english for the evening...
They have a standard range of drinks, and an arrangement with a pizza place that delivers to the pub. There is also a sushi place across the road, and other places to eat close by. Wifi, is turned off for the opera session but otherwise available.
On the Opera side of downtown, near Stortorvet, this is a blues and rock pub. Has some great acts, a Sunday Jam session led by the house band, and Saturday afternoon blues. A long thin type of bar, black inside, it can feel quite odd to come out after the Saturday afternoon blues and walk into a sunny day, although by the door they open the whole wall to the street in good weather.
Following the common pattern, no food, although there is a burger king next door, and real food all around. And there is open wifi.
Down past the town hall near Akershus, on the edge of the red light district, this is a typical Irish Pub. Large, comfortable, with enclosed, sheltered outside seating (in Oslo, this means "place you can have a beer and a cigarette at the same time, even if it is raining"), and a couple of bars. Tuesday night and Saturday afternoon there is a "session" for people who turn up with an instrument, or occasionally just sing a song, and in the front bar there is music most nights, and cover charge on weekends. Galway it isn't, but the place has a nice atmosphere, quiet and loud parts, they have strongbow cider on tap, and room to move.
Per på Hjørnet / Herr Nilsen
These two pubs are next door to each other, together taking up a corner just on the Opera side of downtown. They each have music a couple of nights a week. Both of them are pretty small inside, not the place for claustrophobics but otherwise pretty friendly places, with drinks and wifi to go with your music.
The far end of Grunerlokka, an easy walk from Opera past lots and lots of places to eat. This is a bar downstairs, with a covered "outside" for smokers, and a room upstairs where there are often bands of various kinds. Standard bar, makes a reasonable mojito. (There are a lot of places in Oslo that don't...)
Some other places people go for music:
Rockefeller - somewhere between there and Muddy waters. Big old theatre, a venue for big name bands.
Andy's - opposite parliament. Sports bar with a piano, loud, young crowd
Last Train - behind the hard rock café on Karl Johan's gate. Sometimes has bands. Small dark blue bar. (Not called "Blue train" as I wrote initially - CMN)