陳亮 @ instructables.com shows us his ATtiny85 OLED display build on a breadboard:
This instructable show how to use an ATtiny85 to create a tiny watch core that can run over 1 year before recharge or replace battery.
The instruction to make the watch case may be in next instructable.
I have made a few Arduino watch before, but I found the watch using ATmega boards or ATmega chips are a little bit too big to wear. So I would like to make it with a smaller chips.
ATtiny OLED Watch Core – [Link]