The Casio F-91W is a classic digital wristwatch that’s been in constant production since it was first released in 1989. Ollee Watch offers free Android and iOS apps for syncing data and configuring settings. Even after replacing the guts of the watch with modern hardware, the Ollee Watch isn’t exactly a smart watch in the same way that an Apple Watch, Google Wear OS, or even a Pebble watch is. As I mentioned above, the Ollee Watch isn’t the first mainboard replacement for the Casio F-91W. That project has since been discontinued, but the developer behind the Ollee Watch took inspiration from both (and a bunch of other places) when designing the Ollee Watch project.