You will need to use the MSM tool on a Windows machine.
You should find one or two guides …
- https://forum.xda-developers.com/t/op8t-oos-kb05aa-ba-da-unbrick-tool-to-restore-your-device-to-oxygenos.4180837/
- OnePlus Community
I won’t confuse you with a general guide – please be sure to read thoroughly more than one guide so that you fully understand the workflow. There may be some threads on the forum covering the subject for another insight.
Good luck