We report on reflection absorption infrared spectroscopy (RAIRS) investigations into the influence of oxygen on the surface chemistry of NH3 on Fe{111}. Pre-adsorption of oxygen is found to strengthen the interaction between ammonia and the surface, albeit at the lowest oxygen coverage examined the effect is only notable when the ammonia coverage becomes high. At higher oxygen coverage, the same effect is observed even with very low ammonia coverage. In cases where the oxygen overlayer is ordered, the effects are seen quite clearly, but for disordered overlayers the surface heterogeneity makes assignment of absorption features more difficult; nevertheless, comparison with the ordered examples allows us to identify the same underlying behaviour. When potassium is coadsorbed with oxygen, the effect on ammonia adsorption is threefold, including potassium-dominated and oxygen-dominated features, alongside features that suggest either cancelling or absent influences from the atomic adsorbates. Synthesis of ammonia from nitrogen adatoms in 0.6 mbar H2 shows clear evidence of very similar surface interactions.