I got the phone! And lo and behold it's build 14_161107. (But just wait...)

Anyway, I thought I could just go ahead and install the "particular" TWRP file mentioned here:
https://wiki.merproject.org/wiki/Ada...ishOS_for_onyx

After OEM-unlocking I boldly went ahead.

So far, so good. But what do I do next. When I run "sudo fastboot reboot" the phone starts Android again. When I power off and, then, hold down POWER and VOL DOWN (i.e. the lower one), I get what looks like the stock recovery (1+ logo plus a choice of Chinese or English).

A little searching gave me this idea (after repeating the command above):

Failed...

So, I checked "about phone" again to find the OxygenOS version, which is 3.1.4.

Does that mean I have to take the scenic route and install the current TWRP first?

If so, could you please help me with these stupid questions (sorry for asking again):

1) Is the oos2.2.1-x.zip you mentioned identical with the OnePlusXExtraPartitions-2.2.1-X-0.zip I downloaded from https://forum.xda-developers.com/sho...91&postcount=1 (I got there through the link you provided - https://forum.xda-developers.com/sho...91&postcount=1 )? I know it probably is the one you meant. But I just want to be absolutely sure not to cause damage by accidentally having picked the wrong file.

2) For step 5 do I use the TWRP version linked to from the merproject's wiki https://forum.xda-developers.com/dev...t/dl/?id=15644 or the one from the general instructions here https://forum.xda-developers.com/sho...3&postcount=45 ? I assume it's the merproject's one.

3) Do I need to adjust any settings in TWRP at any point, like erasing parts? There was something about this in the LinearOS instructions, but it seemed confusing to me.

4) Between steps 3 and 4 in nieldk's instructions (3. flash the oldbootloader (OOS2) - 4. Reboot to recovery again (from TWRP menu)): Do I need to boot into Android? I assume not.

I am grateful for any help you can offer!