git@github.com: Permission denied (publickey).

[Al Tabq]

I'm trying to push to my remote git on GitHub using SSH but keep receiving the error: 

Pushing to git@github.com:foobar.git

git@github.com: Permission denied (publickey).

fatal: Could not read from remote repository.

I'm using OAuth and SSH to access Github via SourceTree. The SSH Key from the Account Settings in SourceTree is added in the "SSH and GPG Keys" settings on Github. Am I missing something obvious to get this to work?

Any help would be appreciate.