Antibacterial 1-oxadethiacephalosporins of the formula: ##STR1## [wherein Ar is ##STR2## (in which Acyl is organic or inorganic acyl); COB.sup.1 and COB.sup.2 each is carboxy or protected carboxy;Het is ##STR3## (in which COB.sup.3 is carboxy or protected carboxy); and Y is hydrogen or methoxy;and when COB.sup.1, COB.sup.2, and/or COB.sup.3 is carboxy, pharmaceutically acceptable salts thereof are included, but with a proviso that when Y is methoxy, Het is ##STR4## a pharmaceutical or veterinary composition comprising the said 1-oxadethiacephalosporins and pharmaceutical carrier, and a method for treating or preventing human or veterinary infectious diseases comprising administering the said 1-oxadethiacephalosporin.