The effects of preadsorbed atomic oxygen on nitric oxide (NO) structure and reactivity on Mo(110) are studied via temperature programmed reaction, high-resolution electron energy loss spectroscopy, and infrared reflectance absorbance spectroscopy. NO reaction on two different oxygen overlayers-a saturated low-temperature surface overlayer and a thin-film oxide-is studied in detail. The dissociation of NO to atomic nitrogen and oxygen, the predominant pathway for NO reaction on clean Mo(110), is inhibited by surface oxygen, even though NO dissociation displaces surface oxygen from high- to low-coordination sites. The same low-temperature pathways observed for N-N bond formation on clean Mo(110)-N2O formation from dinitrosyl coupling and N-2 formation from reaction of molecular NO with atomic nitrogen-are observed on the oxygen-modified surfaces, but in lesser relative and absolute amounts than on clean Mo(110). As oxygen coverage is increased, NO desorption becomes the dominant reaction pathway and occurs at increasingly higher temperatures. Vibrational spectroscopy is used to correlate desorption features with distinct NO species, which vary qualitatively with oxygen coverage. We find that, in contrast to earlier studies on other oxygen-modified transition metal surfaces, NO desorption temperature cannot be correlated with the strength of the metal-NO interaction as judged by the internal N-O stretch frequency.