Nitrogen constitutes a significant defect in silicon (Si) as it can affect its mechanical properties. Substitutional nitrogen defects can be created from interstitial nitrogen defects during cooling of the crystal. In turn, the substitutional nitrogen atoms can form micro-defects in Si, most important of which are the interstitial oxygen atoms since they have an impact on a device's characteristics. In the present paper, we study the structure of the defect clusters in Si consisting of substitutional nitrogen and interstitial oxygen atoms. We find that the most stable structure is the one containing two interstitial oxygen atoms, while the presence of these interstitials results to narrow-gap semiconductors.