On first reading I had assumed that OPs must have visited Oslo one or more times in the past. Why else would they leave town if they only have a day plus to enjoy Oslo?
One would have to run through the Rick Steves highlight list to do a "check off" tour in 36 hours given that they are not open at night:
At a Glance
▲▲▲ City Hall Oslo's artsy 20th-century government building, lined with huge, vibrant, municipal-themed murals, best visited with included tour.
▲▲▲ National Gallery Norway's cultural and natural essence, captured on canvas.
▲▲▲ Frogner Park Sprawling park with works by Norway's greatest sculptor, Gustav Vigeland, and the studio where he created them (now a museum).
▲▲▲ Norwegian Folk Museum Norway condensed into 150 historic buildings in a large open-air park.
▲▲ Norwegian Resistance Museum Gripping look at Norway's tumultuous WWII experience.
▲▲ Viking Ship Museum An impressive trio of ninth-century Viking ships, with exhibits on the people who built them.
▲▲ Fram Museum Captivating exhibit on the Arctic exploration ship.
▲▲ Kon-Tiki Museum Adventures of primitive Kon-Tiki and Ra II ships built by Thor Heyerdahl.
▲▲ Holmenkollen Ski Jump and Ski Museum Dizzying vista and a schuss through skiing history.
▲ Nobel Peace Center Exhibit celebrating the ideals of the Nobel Peace Prize and the lives of those who have won it.
▲ Opera House Stunning performance center that's helping revitalize the harborfront.
▲ Akershus Fortress Complex and Tours Historic military base and fortified old center, with guided tours, a ho-hum castle interior, and a couple of museums (including the excellent Norwegian Resistance Museum, listed above).
▲ Norwegian Holocaust Center High-tech walk through rise of anti-Semitism, the Holocaust in Norway, and racism today.
▲ Norwegian Maritime Museum Dusty cruise through Norway's rich seafaring heritage.
▲ Edvard Munch Museum Works of Norway's famous Expressionistic painter.
▲ Grünerløkka Oslo's bohemian district, with bustling cafés and pubs.