Casio's GA-2100 launched in 2019 without any major new technology — just a redesigned octagonal bezel. Watch enthusiasts noticed the resemblance to the Audemars Piguet Royal Oak instantly, and the nickname "CasiOak" spread fast enough that it's now how most people refer to the watch at all. Demand got so intense that a whole cottage industry of metal mod kits sprang up for people chasing an even closer Royal Oak look.

This GA-2100-1A1JF is the original matte black colorway that started it all — carbon-fiber-reinforced resin case, 20-bar water resistance, and the same shock-resistant structure G-Shock has built its name on since 1983.