OXA-24 is a carbapenem-hydrolyzing class D β-lactamase (CHDL) that poses a serious medical threat by destroying carbapenem class antibiotics. OXA-160 is a clinically-derived OXA-24 variant with a Pro→Ser substitution. Previously, it was shown that OXA-160 has higher catalytic activity against third-generation cephalosporins compared to OXA-24 and is able to maintain normal activity against penicillins and carbapenems. To slow deacylation, we introduced a second mutation (Val130Asp) to allow us to capture a drug-complex structure. We examined the OXA-160/Val130Asp variant in complex with the substrates cefotaxime, ceftazidime, and aztreonam using X-ray crystallography. Our analysis shows that all three of these bulky antibiotics require β5-β6 and/or omega loop deviations, and we propose that these conformational changes are made possible by replacing the restricted proline with the more flexible serine. These crystallographic structures reveal that a Pro227Ser mutation enlarges the active site, better accommodating advanced cephalosporin drugs.