The present invention provides vaccine compositions comprising OmpA, or antigenic fragments thereof, and related methods of active immunization against A. baumannii infection. The invention also provides antibodies and antigen-binding parts thereof that specifically bind to OmpA, and related methods of passive immunization against A. baumannii infection. The compositions and methods of the invention are useful for preventing or treating A. baumannii infections, including those caused by strains resistant to carbapenems and all other antibiotics except colistin or tigecycline, also referred to as extreme drug resistant (XDR) A. baumannii infections, and those resistant to every FDA approved antibiotic, also referred to as pan-drug resistant (PDR) A. baumannii infections.