Oscar is the only person who could be Red John. Why? I have 2 different
reasons. One, internal to the plot of the show: Red John has a strong
network of followers in all levels of California Law Enforcement. Oscar
is the Assistant District Attorney, so he is at the top of the law
enforcement food chain. Also he has access, through normal legal
channels, to lots of information on Patrick, the Red John case, and any other criminals he may find useful. A monster like Red John has a very large ego, so in his professional life he would be in a place of prominence and authority.

Reason
Two: this one steps outside the plot and looks at things from the Point
of View of the writers. Whom does a Mystery writer always want the Bad
Guy to turn out to be? The last person you would expect of course, as
well has someone who has been under your nose the whole time. How many
mysteries have concluded with the Bad Guy being either the person who
hired the detective, or the Sheriff who has been working with the
detective from the very start? Red John needs to be a character who has
been there, on the show, since very early on. Most of the people on
Patrick's list showed up later. I don't remember what episode Oscar
first appeared, but he has been there, in the background of California
Law Enforcement, from the very beginning. Also, aside from Lisbon's
team, I believe he is the only character left on the show who has been
there that long!

Lorelai Martins stated that Patrick has shaken
hands with Oscar. My theory is that she was mistaken.. perhaps Oscar
shakes hands with everyone, so she assumed they had shaken, but I'll be if we check the tapes Oscar and Patrick have never shaken hands.

Additionally, while there is not much evidence to go off of, I believe it is clear that Oscar is the kind of manipulative guy that Red John will turn out to be.

On a side note, I floated the idea that Patrick was Red John for a while, but more than once we know exactly where Patrick was when Red John was killing somebody.