According to a report I found the Head of Communication Joachim Westher Andersen at Oslo Airport said in 2015:
We know that free drinking water is popular, and we now have several fountains we hope the passengers will appreciate. People can bring empty water bottles through the security check and refill their water after getting through”
Immediately after the security there is a two drink stations. In the Domestic part of the terminal, you can refill your water bottle by gate A10, B2 and C4. On International flights you can refill bottles at the gate D4 and E12.
All filling stations are marked with signs.
and by the way he said free WiFi as well !
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