  We examine the effects of SMBH feedback on the CGM using a cosmological
hydrodynamic simulation \citep[{\sc Romulus25};][]{Tremmel2017} and a set of
four zoom-in `genetically modified' Milky Way-mass galaxies sampling different
evolutionary paths. By tracing the distribution of metals in the circumgalactic
medium (CGM), we show that \ion{O}{6} is a sensitive indicator of supermassive
black hole (SMBH) feedback. First, we calculate the column densities of
\ion{O}{6} in simulated Milky Way-mass galaxies and compare them with
observations from the COS-Halos Survey. Our simulations show column densities
of \ion{O}{6} in the CGM consistent with those of COS-Halos star forming and
quenched galaxies. These results contrast with those from previous simulation
studies which typically underproduce CGM column densities of \ion{O}{6}. We
determine that a galaxy's star formation history and assembly record have
little effect on the amount of \ion{O}{6} in its CGM. Instead, column densities
of \ion{O}{6} are closely tied to galaxy halo mass and BH growth history. The
set of zoom-in, genetically modified Milky Way-mass galaxies indicates that the
SMBH drives highly metal-enriched material out into its host galaxy's halo
which in turn elevates the column densities of \ion{O}{6} in the CGM.
