A strong oscillation pattern was observed in optical second-harmonic generation (SHG) at a Ag(lll) surface under pyridine adlayers with variable thickness. This oscillation appears only when s-, but not p-, polarized fundamental light is used. This effect is attributed to the modulation of the radiation efficiency of the fundamental light penetrating into the Ag bulk and of the second-harmonic light radiating out to the vacuum by pyridine overlayers through optical interference. A standard optical interference model has been developed to quantitatively analyze the oscillation patterns and determine the adlayer thickness. It was found that at 90 K the pyridine overlayer growth rate is 1.4 Angstrom per Langmuir exposure.