The goal of the study is to identify a dose and schedule of CC-486 that can be safely administered with R-CHOP. To evaluate the safety and maximum tolerated dose (MTD) or the maximal administered dose (MAD) of CC-486 in combination with rituximab, cyclophosphamide, doxorubicin, vincristine, and prednisone (R-CHOP) in subjects with high risk (IPI 2 or more) previously untreated DLBCL or Grade 3B FL.