A positive parity sequence of $\Delta I=2$ γ transitions has been identified above $I^{\pi }=9/2^{+}$ state ($E_x=2019$ keV) in $^{115}$Sb through in-beam γ ray spectroscopic technique. Rotational features of this sequence are found similar to a low-K decoupled band. Observation of this newly identified low-K decoupled band, along with the earlier reported strongly coupled high-K band in this nucleus, provides the first experimental evidence for prolate-oblate shape coexistence associated with $g_{9/2}$ proton-hole configuration around $Z=50$ shell closure. Experimental results are reproduced reasonably well in the frameworks of the projected shell model and the total Routhian surface calculations.