I have bought the boat tour at the railway station but they are advertised every where in the city so I guess you won’t have any difficulties finding the one that convenient you the most. I took the first one in the morning and started the day in a veg bar nearby the port.
Personally I wouldn’t do it again because the Fjord of Oslo is not exactly what I had in my mind but I would come at any time in this wonderful city and would certainly take another tour of the Fjords, but in the Northern part of the country.
However it has been relaxing and entertaining.
I am so addicted to the sea that I never miss an oppurtunity to catch a ferry or other embarcations!
Then the sun has accompanied us for the whole tour. I think I couldn’t ask for more!