The diversity and evolution of the class A OXY β-lactamase from Klebsiella oxytoca were investigated and compared to housekeeping gene diversity. The entire blaOXY coding region was sequenced in 18 clinical isolates representative of the four K. oxytoca β-lactamase gene groups blaOXY-1 to blaOXY-4 and of two new groups identified here, blaOXY-5 (with four isolates with pI 7.2 and one with pI 7.7) and blaOXY-6 (with four isolates with pI 7.75 and three with pI 8.1). Genes blaOXY-5 and blaOXY-6 showed 99.8% within-group nucleotide similarity but differed from each other by 4.2% and from blaOXY-1, their closest relative, by 2.5% and 2.9%, respectively. Antimicrobial susceptibility to β-lactams was similar among OXY groups. Nucleotide sequence diversity of the 16S rRNA (1,454 bp), rpoB (940 bp), gyrA (383 bp), and gapDH (573 bp) genes was in agreement with the β-lactamase gene phylogeny. Strains with blaOXY-1, blaOXY-2, blaOXY-3, blaOXY-4, and blaOXY-6 genes formed five phylogenetic groups, named KoI, KoII, KoIII, KoIV, and KoVI, respectively. Isolates harboring blaOXY-5 appeared to represent an emerging lineage within KoI. We estimated that the blaOXY gene has been evolving within K. oxytoca for approximately 100 million years, using as calibration the 140-million-year estimation of the Escherichia coli-Salmonella enterica split. These results show that the blaOXY gene has diversified along K. oxytoca phylogenetic lines over long periods of time without concomitant evolution of the antimicrobial resistance phenotype.