So depends on what you're looking for. If you're after 'vintage' clothes or that aesthetic - try the Uff at jarnbanetorget ( I believe they opened another one near Gunerius as well but I haven't been myself), Velouria vintage (expensive!), Robot (also expensive), or Aces.
If you're in the market for cheap clothes instead of fashion try the M&E Secondhand at either Grensen or Økernveien. Their prices seem to be the most reasonable, seeing as so many of the second hand shops in Oslo bank on the thrifting trend and charge over the moon prices. You might also get lucky with the various fretex shops, but be weary of the ones in more 'hip' areas, as they also seem to cater to the vintage hunting crowd.
Good luck!