OAuth only does AuthZ. I've always found OAuth more complicated because you have to combine it with other technologies to get AuthN
AuthN: Authentication (who you are) AuthZ: Authorization (what you are allowed to do)
I've never had to clarify what someone is actually trying to accomplish when they want "SAML 2.0"
Since OIDC is better than SAML, which is probably the scariest security standard on the Internet, I think it's worth being clear to people that OIDC/OAuth is viable.
The SAML authz story, for what it's worth, is pretty shady.
There's a lot of functional overlap between SAML and OIDC/OAuth, but SAML is a very different (and idiosyncratic) protocol; the "what" is the same, but the "how" is very different.
OAuth is way more complex, I've used it countless times and still get confused by it. It has more complex patterns like having a separate resource server and authentication server, it's used for more purposes, e.g. sometimes for API access and sometimes for login and sometimes a confusing mix of both, and there are big differences between v1 and v2 and some services are still using v1.
I once tried to implement it, and found that the specification was spread across ~500 pages of dense PDFs. I find it to be complex.