Using the density functional theory-individual gauge for localized orbitals (DFT-IGLO) method, the N-15 shielding for a nitrosyl ligand bound to a cobalt in a tetraphenylporphyrin (TPP) complex is studied. The effects of changing the Co-N-O bond angle, N-O and Co-N bond lengths, and the displacement of the cobalt atom from the porphinato core on the nitrosyl N-15 Shielding tensor have been investigated. Results indicate a great sensitivity of the nitrosyl N-15 Shielding to local geometry.. Using an experimental geometry, qualitative agreement between calculated and observed values is achieved. The presence of a swinging motion of the nitrosyl ligand is also discussed.