Oxidation of genomic DNA forms the guanine lesion 7,8-dihydro-8-oxoguanine (8-oxoG). When in the template base position during DNA synthesis the 8-oxoG lesion has dual coding potential by virtue of its anti - and syn -conformations, base pairing with cytosine and adenine, respectively. This impacts mutagenesis, because insertion of adenine opposite template 8-oxoG can result in a G to T transversion. DNA polymerases vary by orders of magnitude in their preferences for mutagenic vs. error-free 8-oxoG lesion bypass. Yet, the structural basis for lesion bypass specificity is not well understood. The DNA base excision repair enzyme DNA polymerase (pol) β is presented with gap-filling synthesis opposite 8-oxoG during repair and has similar insertion efficiencies for dCTP and dATP. We report the structure of pol β in binary complex with template 8-oxoG in a base excision repair substrate. The structure reveals both the syn - and anti -conformations of template 8-oxoG in the confines of the polymerase active site, consistent with the dual coding observed kinetically for this enzyme. A ternary complex structure of pol β with the syn -8-oxoG: anti -A Hoogsteen base pair in the closed fully assembled preinsertion active site is also reported. The syn -conformation of 8-oxoG is stabilized by minor groove hydrogen bonding between the side chain of Arg283 and O8 of 8-oxoG. An adjustment in the position of the phosphodiester backbone 5′-phosphate enables 8-oxoG to adopt the syn -conformation.