The Oathpact only binds the Fused to Braize and doesn't directly affect Odium. Odium is bound by Honor and Cultivation.
And having to renew the Oathpact would mean that the protagonists are desperate and losing and need a stop-gap measure, which I feel is a bad way to end Arc 1.
I think the first series will deal with the events on Roshar--the Fused, the Listeners, the Radiants, Urithiru, etc. Series 2 will deal with Odium, the greater Roshar system, and a different set of challenges to the characters.