A density functional theory has been applied to study adsorption from oligomer monomer solutions on solid surfaces modified with end-grafted polymers. Chains are modeled as freely jointed tangent spheres. All spherical species interact via Lennard-Jones (12-6) potential. The grafted chains are not attracted by the surface. Fluid molecules interact with the substrate via Lennard-Jones (9-3) potential. It is shown how a shape of the relative excess adsorption isotherms depends on such parameters as the oligomer length, the grafting density, oligomer substrate interactions, oligomer grafted chains interactions, and oligomer monomer interactions in the bulk solution. As attractive interactions between free and grafted chains strengthen, the relative excess adsorption isotherm can change from negative, through S-shaped with an adsorption azeotropic point, to positive in the whole concentration region. A change of oligomer monomer interactions causes the opposite effects. For low grafting densities of attractive bonds, the relative excess of oligomers increases, reaches a maximum, and decreases. The structure of surface layers is also analyzed.