This is an open-label, Phase 1b dose escalation and expansion study of OMP-305B83 plus FOLFIRI or FOLFOX to evaluate the safety, efficacy, and pharmacokinetics of OMP-305B83 in combination with FOLFIRI or FOLFOX in patients with metastatic Colorectal Cancer. This study consists of a screening period, a treatment period, and a post-treatment follow-up period in which patients will be followed for survival for up to 5 years. Patients will be enrolled in two stages: a dose-escalation stage and an expansion phase.