Dear customer, the P8 Max has 2 sim slots - Slot1 for a nano-sim or a micro-SD card and Slot2 for a micro-sim card. If you wish to use 2 nano-sims at the same time, you will have to use a sim card holder to convert one nano-sim card into micro-sim size. You can put in a O2 sim but it will have only access to 2G and 3G networks.