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KM there's "OEM unlocking" it must be enabled so you can later, with working Android Tools, enter the command "fastboot oem unlock"
Thanks, this piece of information was missing on that webpage!
KM t might be useful to know which OxygenOS version you are on currently
The OxygenOS version is OP3_O2_open_25. Android version is 8.0.0 and Android security patch level is September 1, 2017.
I badly hope I won't need to reinstall it, but it is good to know there are options out there. Thanks!