NH3 is a carbon-free fuel with good storage and transport characteristics. Applying Moderate or Intense Low-oxygen Dilution (MILD) combustion to NH3/syngas blended fuel is expected to achieve clean and low-carbon combustion. This work explores the impacts of changing the NH3/syngas ratio and O2 content in co-flow on the combustion characteristics and the NOx formation and transformation mechanisms. As the proportion of NH3 increases, the flame is gradually lifted, while the rise of H2 in syngas reduces the lifted height. As the O2 proportion is increased to 15%, almost the entire computational domain is transformed from MILD to high-temperature combustion when the ratio of NH3 to syngas is 3:7. NH that is derived from the decomposition of NH3 participates more actively in the consumption process of NO and N2O. N radicals are more likely to generate N2 when O2 content in hot co-flow increases.