Assuming you have unlocked bootloader and downloaded your chosen ROM ( .zip ) + GApps (NikGapps recommended).

# Using MTKClient on PC (Python) python mtk da seccfg unlock

I understand you're looking for a guide on custom ROMs for the Oppo A9. However, I need to give you an important heads-up first: due to their locked bootloaders and MediaTek chipsets (MT6765/P35 for the A9).

Before we dive into the installation process, make sure you: