Introduction: Bacteriophages have been proven to be effective in controlling bacteria and potential as alternatives to antibiotics. Recently, combination treatments using both bacteriophages and antibiotics have been reported to substantially improve antimicrobial activity when compared with their treatments. Objectives: To isolate a lytic bacteriophage specific to Staphylococcus aureus ATCC 25923 and determine antimicrobial activity against S. aureus ATCC 25923 of the isolated bacteriophage and oxacillin when used individually and in co-treatment. Methods: A lytic bacteriophage was isolated from a hospital wastewater sample by enrichment technique. Its host range was examined using the spot test method. The effect of oxacillin on the lytic activity of the bacteriophage was determined. Lastly, the minimal inhibitory concentrations (MICs) of the bacteriophage and oxacillin against S. aureus ATCC 25923 when used individually and in co-treatment were determined using the spot test method. The fractional inhibitory concentration index (FICI) was used to analyze the co-treatment. Results: A bacteriophage, designated bacteriophage UBU-SA1, was isolated. It was found to be highly specific to S. aureus ATCC 25923. Its lytic activity was not inhibited by oxacillin. MICs of bacteriophage UBU-SA1 and oxacillin against S. aureus ATCC 25923, when used alone, were 105 PFU/mL and 50 __ampersandsignmicro;g/mL, respectively. Co-treatment of bacteriophage UBU-SA1 and oxacillin substantially reduced the MICs of both agents required to inhibit the bacterial host when compared with their treatments. The FICI indicated a synergistic effect between bacteriophage UBU-SA1 and oxacillin against S. aureus ATCC 25923. Conclusion: Bacteriophage UBU-SA1, by itself or in combination with antibiotics, may be useful as a therapeutic agent in controlling S. aureus