  The observation of metallic interface between band insulators LaAlO$_3$ and
SrTiO$_3$ has led to massive efforts to understand the origin of the phenomenon
as well as to search for other systems hosting such two dimensional electron
gases (2-DEG). However, the understanding of the origin of the 2-DEG is very
often hindered as several possible mechanisms such as polar catastrophe,
cationic intermixing and oxygen vacancy (OV) etc. can be operative
simultaneously. The presence of a heavy element makes KTaO$_3$ (KTO) based
2-DEG a potential platform to investigate spin orbit coupling driven novel
electronic and magnetic phenomena. In this work, we investigate the sole effect
of the OV, which makes KTO metallic. Our detailed \textit{ab initio}
calculations not only find partially filled conduction bands in the presence of
an OV but also predict a highly localized mid-gap state due to the linear
clustering of OVs around Ta. Photoluminescence measurements indeed reveal the
existence of such mid-gap state and O $K$-edge X-ray absorption spectroscopy
finds electron doping in Ta $t_{2g}^*$ antibonding states. This present work
suggests that one should be cautious about the possible presence of OVs within
KTO substrate in interpreting metallic behavior of KTO based 2-DEG.
