The interaction between a polymer segment and an oil-water interface is represented by an asymmetric square-well potential where the well-depth on one side reflects water-polymer and the well depth on the other side reflects oil-polymer interactions. The polymer is represented by a Gaussian chain. The polymer's density distribution is calculated along a coordinate perpendicular to the interface. Results are obtained as a function of the well width, the well depth and its asymmetry and, most important, the polymer's length. For a symmetric well, the distribution shows a strong maximum at the interface provided that the polymer is sufficiently long. For an asymmetric well, the polymer is also strongly adsorbed at the interface provided that the polymer is sufficiently long and provided that the larger well-depth does not exceed a critical value that depends on the smaller well-depth. The calculations are in substantial agreement with experimental results that indicate nearly irreversible adsorption of long-chain molecules at an oil-water interface.