We all know how Brandon likes to surprise his readers with unexpected plot twists. The nature of these surprises varies from book to book (Brandon is too imaginative a writer to repeat himself exactly) but all his major works to date have ended with the Big Bad antagonist being someone other than we expected near the start of the story.
I believe we can predict the timing, if not the details, of future Stormlight Archive reveals by looking at how Brandon has paced these things in previous works.
Let's start with Mistborn. My paperback editions are 643, 763 and 724 pages long respectively, = 2130 total.
How about Warbreaker, which is 652 pages?
How about Elantris, which is 615 pages in my edition?
This pacing surprsingly consistent (although not identical) across all three sources:
Average location of first major "whoah, what I thought was going on here is fundamentally wrong" moment is 42% of the way through (the first such moment in any of the three stories was at 29%).
Average location of first unveil of ultimate Big Bad is at 74% (first such moment in any of the three stories is 66%)
But wait a minute...
Stormlight Archive is 10 books. After WoR, we are 20% of the way through. If this series is paced anything like what Brandon has done in the past, we have not yet reached the first significant plot twist, and will not meet the true Big Bad of the epic until book 6 or 7!
And what about the Cosmere as a whole? Brandon says there will be 36 novels total, of which we have seen 7 so far. That's 19%.
From this I conclude that:
Odium is not the Big Bad of Stormlight Archive, let alone the Cosmere as a whole (if he was, this would mean Brandon has unveiled his ultimate enemy a full 46% sooner than in any previous Cosmere works) We ain't seen nothing yet!