We report the discovery of the first type II Cepheids (BL Herculis stars) pulsating solely in the first overtone. We found two such objects among tens of millions of stars regularly observed by the Optical Gravitational Lensing Experiment survey in the Large Magellanic Cloud. Our classification and the pulsation mode identification is based on the position of these stars on the period-luminosity and color-magnitude diagrams and on the light-curve analysis. We discuss why single-mode first-overtone BL Her pulsators must be very rare. For the two discovered stars we present nonlinear models that successfully reproduce their light variation. These models indicate that both first-overtone pulsators should be more massive than is typically assumed for BL Her stars, i.e., their masses should be above 0.75 . However, the higher mass requires higher luminosity to match the observed periods of the stars, which is inconsistent with observations.