@bateske, Do you think an external Mosfet switch to power off the display is really necessary?
According to the OLED datasheet, with the display in software activated sleep mode it will typically draw only 1uA from Vdd and 2uA from Vcc, for 3uA total. The Atmega32u4 can draw as little as 1uA in power down mode, if the watchdog timer is disabled.
Without factoring in any idle power used by the power supply and other circuitry, let's say the total would be 5uA. With a fully charged 180mAh battery that had no internal leakage, you could sleep for over 4 years.
Now, if any circuitry beyond the processor and display draws any significant power while the unit is asleep, then the 3uA sleep current drawn by the display is even less of a factor in overall sleep time.
In other words, is saving 3uA of current draw worth the cost of adding external power down circuitry for the display?