Phytochemical studies of the chloroform soluble fraction of Dioscorea opposita resulted in the isolation of four new compounds, 3,5-dihydroxy-4-methoxybibenzyl (1), 3,3',5-trihydroxy-2'-methoxybibenzyl (2), 10,11-dihydro-dibenz[b,f]oxepin-2,4-diol (3), and 10,11-dihydro-4-methoxy-dibenz[b,f]oxepin-2-ol (4), together with an additional fifteen known compounds. The structures of 1-4 were elucidated by spectroscopic methods including 2D NMR. All of the nineteen isolated compounds were tested in the DPPH, superoxide anion radical scavenging assays and cyclooxygenases (COXs) inhibition assay. Of those, compounds 7, 9, 11, 12, 13, 15 and 18 exhibited radical scavenging activities and compounds 2, 3, 8, 13, 15 and 16 showed selective inhibitory activities against COX-2.