Detection of NO in biological systems and removing or reducing NO for environment protection is paramount. Herein, we investigate the influence of NO and (NO)₂ adsorption on the properties of Fe-N4 porphyrin-like graphene (G-Fe-N4) sheets using periodic DFT calculations with the dispersion correction. The results show that NO can be converted into N₂O through adsorbed (NO)₂ with a total energy barrier of 0.92 eV. The adsorption of N₂O and of two NO on O/G-Fe-N4 sheets can proceed through (N₂)_gas + (O₂)_ads and (N₂O)_gas + (O₂)_ads, respectively. Both paths have a rate-determining step with a high (∼1.80 eV) energy barrier. Nevertheless, the formation of (O₂)_ads on the G-Fe-N4 can be regarded as an oxygen reduction reaction (ORR) precursor. Detailed analyses of the electronic properties of the various systems involved in this reaction reveal the increased spin filter characteristics for some structures. Hence, the obtained spin filter parameters of the NO@G-Fe-N4 and (NO)₂@G-Fe-N4 structures are 72.53% and 47.96%, respectively. Also, it is found that the adsorption of NO gas molecules induces different energy antiresonant dips not found in G-Fe-N4, which are induced by quasi-bound states related to the adsorbate and Fe-N4 defect.