How is OAuth centralizing auth? It's generally used for one of two things: 1) Single Sign-On - something that generally increases the security of applications under one organization, where authentication has to be shared one way or another, and 2) "social login" - something that takes a website from being its own and only auth provider to supporting multiple external providers.

2) is the exact opposite of centralization and 1) is basically equivalent to dynamic linking which, while "centralization" in theory, is generally considered a good security practice.