This is a Phase 2 multicenter, randomized, parallel arms, double-blind study of vanucizumab to evaluate the efficacy and safety of vanucizumab in combination with oxaliplatin, folinic acid, and 5-fluorouracil (5-FU) (mFOLFOX-6) versus bevacizumab (Avastin) + mFOLFOX-6 in participants with previously untreated metastatic colorectal cancer (mCRC). The study consists of 2 parts: a safety run-in open-label, single-arm part (Part 1) and a randomized, parallel-arms, double-blind part (Part 2). During Part 1 at least 6 eligible participants will receive 2000 milligrams (mg) vanucizumab every 2 weeks + mFOLFOX-6 in order to confirm the dose and schedule that will be used in Part 2. In Part 2, all eligible participants will be randomized in a ratio of 1:1 to receive either mFOLFOX-6 + vanucizumab or mFOLFOX-6 + bevacizumab. Study treatment (induction and maintenance) will be given on Day 1 of each 14-day cycle. Induction therapy will consist of up to 8 cycles of mFOLFOX-6 plus either bevacizumab or vanucizumab. Maintenance therapy will consist of 5-fluorouracil and folinic acid plus either vanucizumab or bevacizumab for up to 24 months or until disease progression, unacceptable toxicity, Investigator decision or consent withdrawal, whichever occurs first.