Just to be clear since "oppression" is a very broad term: the Navajo (and most other Native American tribes) are victims of genocide. It was a far, far, far more systematic destruction effort than mere marginalization.

Children were stolen, forbidden from learning their native language, killed en masse, food supplies were destroyed, land was continuously taken from them the second anything valuable was discovered on it, etc. etc.

It's really horrific stuff and the effects are still extremely clear on the reservations today.