  We observe an anomalous oxygen isotope effect on the a-axis component of the
far-infrared electronic response of detwinned YBa2Cu3$^{16,18}$O$_{6.9}$. For
$^{18}$O a pronounced low-energy electronic mode (LEM) appears around 240
cm$^{-1}$. This a-axis LEM exhibits a clear aging effect, after one year it is
shifted to 190 cm$^{-1}$. For $^{16}$O we cannot resolve a corresponding a-axis
LEM above 120 cm$^{-1}$. We interpret the LEM in terms of a collective
electronic mode that is pinned by `isotopic defects', i.e. by the residual
$^{16}$O in the matrix of $^{18}$O.
