The O2 Xda brand was a range of Windows Mobile PDA phones, marketed by O2, developed by O2 Asia and manufactured by multiple OEMs (mainly HTC, Quanta and Arima). The first model was released in June 2002. The last models came to market in 2008. The "X" represents convergence of voice and information/data within one product; the "DA" stands for "Digital Assistant", as in PDA. The name of XDA Developers is derived from it.

A Pocket PC is a class of personal digital assistant (PDA) that runs the Windows Mobile or Windows Embedded Compact operating system that has some of the abilities of modern desktop PCs. The name was introduced by Microsoft in 2000 as a rebranding of the Palm-size PC category. Some of these devices also had integrated phone and data capabilities, which were called Pocket PC Phone Edition. Windows "Smartphone" is another Windows CE based platform for non-touch flip phones or dumber phones.