The Paranoid Android team began working on the Android 13 version of their ROM shortly after the release of the OS in August 2022. Within a few months, they dropped the first beta build for several phones, including the OnePlus 9 series. This gave users a decent OxygenOS 13 alternative to try on their phones. But since the ROM was in beta, it was filled with bugs and issues. Now, nine months after the first Paranoid Android beta was released, the ROM has gone stable for OnePlus' 2021 flagship phones.
