What are the best ways to learn Oxygen as a beginner?

The best way is by watching the videos from the Official Oxygen Youtube Channel :
https://www.youtube.com/channel/UCtG-mJIfI5Kq5wp4X1RQOXg

And the Official Oxygen Documentation : https://oxygenbuilder.com/documentation/

There are also some 3rd party video courses (free and premium)

Permaslug Course:
https://permaslug.com/ultimate-oxygen-course/

Permaslug YouTube:
https://www.youtube.com/channel/UCofbAeeBdTTZgeHinzl5u6A

Isotropic Design YouTube: https://www.youtube.com/channel/UCJspEIo2HrjfaDMMXETluYg

Stratos Tutorials YouTube:
https://www.youtube.com/channel/UCr-o6Wusaqh3tkVFAtyZKAA

Kevins' videos:
https://www.youtube.com/channel/UC4KXtFds_3R7xwGV79rZysg