Narrow Absorption Components (NACs) have been detected in essentially all unsaturated P Cygni profiles of O-type stars (Howarth & Prinja, 1989). Time-series of observations have revealed their migration through the profile's absorption trough (Prinja et al., 1987, Henrichs et al., 1988). The interpretation of the NACs is essential for the understanding of the dynamic structure of the winds of O stars and of the driving mechanisms behind it. The P Cygni profile substructures caused by isolated reverse and forward shocks were calculated in a ‘line only’ approximation for a spherically symmetric, isothermal stellar wind. The Comoving Frame method was used for an accurate treatment of the complex radiation transfer in the shock environment. The underlying velocity structure follows a β-law (except in the innermost layers where a polynomial law was used) while the ionization was taken to be constant throughout the entire wind.