Read before you flash

Warnings

Hard brick warning

Missing any of the points below can hard brick your device — the only solution then is a service center.

For OOS 15.0.0.1301 users

1.
Update 1301 with a future OOS OTA update with a locked bootloader, or flash 1301 on the other slot too (same as the reverting method) — and only then flash custom ROMs shipped without firmware. This way the other slot containing old firmware (previous OTA update 1201) gets overwritten with the 1301 one.
2.
DO NOT flash custom ROMs or downgrade to previous ROMs with older firmware. Be careful of the other slot as well — read point 1 again.
3.
DO NOT update OxygenOS with an unlocked bootloader.

Reverting back to stock OOS 15.0.0.1301 has been tested by a 1301 user (@shaktiraj on Telegram). Your only possibility is reverting on this same version (1301) — or, on future updates, reverting on the same version you flashed ROMs from.

If you never updated to OOS 15.0.0.1301

Neither update to OOS 15.0.0.1301 if you haven't already, nor revert back to this version from custom ROMs. Other than that, you can use your device as you wish.

For newcomers

1.
Go through the warnings above, the guides, and the tutorial videos for ziti before you begin.
2.
Before flashing, make sure proper adb/fastboot drivers are installed on your PC.
3.
Downgrading is dead for OOS 15.0.0.1301+ users. If you haven't updated, do NOT downgrade to OOS 13.1 in an unlocked-bootloader state — OOS 13.1's bootloader is corrupt.

Flashing steps for 1301/1601+ users

How to flash custom ROMs on 1301+

You'll be needing

OOS 15.0.0.1301 / 1601 OTA file ↗
ADB & fastboot drivers ↗
Recovery and files ↗

Step 1Fill the other slot with your current firmware

Method 1 — Local install

Download the full OTA of the same version (not a downgraded version) you are on (1301/1601/1604/1800+) and use Local install from the updater to install it.

— or —

Method 2 — Reverting method

Flash recovery first, then flash the 1301/1601 same-version stock ROM you are on (just like reverting from custom ROMs to OxygenOS) to fill the other slot with this same version firmware.

1.
Flash recovery from bootloader.
2.
Format from recovery.
3.
Sideload 1601 from adb.
4.
If it asks “verification fails, install anyway?” → Yes.
5.
After the sideload reaches 47% it asks for rebooting to recovery to flash the additional file → NO.
6.
Reboot to bootloader from Advanced.
7.
Flash the my_shit script ↗.

Your other slot will now reboot into this same OOS.

Step 2Now you can flash custom ROMs

1.
Flash recovery again.
2.
Format from recovery.
3.
Sideload the custom ROM.
4.
Reboot to system.
✓
Done.

Tested by @r0ckstar126

Support

Flash at your own risk — the community is not responsible for any of your mishaps.