Looking at the Island ferries map and the zones map of the Oslo public transport system, it isn't entirely clear which zone the islands are in because they aren't shown on the simplified zones map. I would guess that (shown by Google Maps as being in Akershus) is in zone 2S, and the remainder in zone 1 but it would be great if anybody could confirm that.
The zone borders do not follow the province borders and all islands served by the Ruter ferries (the public transport operator) are in zone 1. Also on the mainland, zone 1 extends beyond the border between Oslo and Akershus. Nesoddtangen (not an island, but also served by the ferries) is in zone 2S, but that is obvious on the zone map.
You can find the current time tables for the island ferries here, where all stops are listed with the corresponding tariff zone.
You can also use the journey planner, where the tariff zones of all stops are listed, and where you can also find a direct link to the fare information and different types of tickets for each suggested journey.