Mass-loss rate, , is one of the key parameters affecting evolution and observational manifestations of massive stars and their impact on the ambient medium. Despite its importance, there is a factor of ∼100 discrepancy between empirical and theoretical of late-type O dwarfs, the so-called weak-wind problem. In this Letter, we propose a simple novel method to constrain of runaway massive stars through observation of their bow shocks and Strömgren spheres, which might be of decisive importance for resolving the weak-wind problem. Using this method, we found that of the well-known runaway O9.5 V star ζ Oph is more than an order of magnitude higher than that derived from ultraviolet (UV) line fitting and is by a factor of 6–7 lower than those based on the theoretical recipe by Vink et al. and the Hα line. The discrepancy between derived by our method and that based on UV lines would be even more severe if the stellar wind is clumpy. At the same time, our estimate of agrees with that predicted by the moving reversing layer theory by Lucy.