Currently, there is released by Xiaomi for OPPO devices. Running MIUI 8 on an OPPO F1s requires installing an Unofficial Port created by third-party developers. The most common ports available are based on the MediaTek architecture of the device.
Oppo F1s has limited internal storage. Place the .zip file on an external microSD card. Wiping internal storage is part of the process.
Since there is no official MIUI 8 update for the Oppo F1s, users must rely on ported by the developer community. Oppo F1s - Full phone specifications - GSMArena.com