Official firmware ships with Android 5.1.1 (Lollipop) with ColorOS 3.0 or Android 6.0 (Marshmallow) depending on the regional update.
You cannot use Odin (Samsung) or MiFlash (Xiaomi). MediaTek devices require: oppo a37m mt6755 firmware
Before starting, ensure you have the following components prepared: Official firmware ships with Android 5