In this paper, we report a wireless gas sensor based on surface acoustic waves (SAW). For room temperature detection of oxygen gas, a novel nanostructured Zn_xFe_yO gas-sensitive film was deposited on the surface of a SAW resonator by an oblique magnetron co-sputtering method. The measurements of X-ray diffraction (XRD) and a scanning electron microscope (SEM) showed that the crystal phase composition and the microstructures of Zn_xFe_yO films were significantly affected by the content of Fe. The experimental results showed that the sensors had a good response to O₂ at room temperature. The max frequency shift of the sensors reached 258 kHz as the O₂ partial pressure was 20%. Moreover, X-ray photoelectron spectroscopy (XPS) was performed to analyze the role of Fe in the sensitization process of the Zn_xFe_yO film. In addition, the internal relationship between the Fe content of the film and the sensitivity of the sensor was presented and discussed. The research indicates that the nanostructured Zn_xFe_yO film has a good potential for room temperature O₂ gas detection applications.