Instructions how to use can be found: HERE
There is only one difference. To boot into Qualcomm download mode you need to press Volume up + Volume down when phone is powered off.
There is also a guide on YouTube. --> LINK
You can easily go to OxygenOS with official Updater app.
Just download OxygenOS OTA.zip and choose it with updater.
After reboot i personally would factory reset the phone.
Thread for OTA.zip:
Oneplus 7 Repo Oxygen OS builds
Feel free to report about your experience!
@Titokhan - for the Update.zip files
@Thyson - for testing the MSM Tool and reporting
@jmax91 - for the tip (tool)
@LloydSmallwood - for the unbrick guide