  We present the first detection in space of O-protonated carbonyl sulfide
(\ch{HOCS+}), in the midst of an ultradeep molecular line survey toward the
G+0.693-0.027 molecular cloud. From the observation of all $K$$_a$ = 0
transitions ranging from $J$$_{lo}$ = 2 to $J$$_{lo}$ = 13 of \ch{HOCS+}
covered by our survey, we derive a column density of $N$ = (9 $\pm$
2)$\times$10$^{12}$ cm$^{-2}$, translating into a fractional abundance relative
to H$_2$ of $\sim$7$\times$10$^{-11}$. Conversely, the S-protonated \ch{HSCO+}
isomer remains undetected, and we derive an upper limit to its abundance with
respect to H$_2$ of $\leq$3$\times$10$^{-11}$, a factor of $\geq$2.3 less
abundant than \ch{HOCS+}. We obtain a \ch{HOCS+}/OCS ratio of
$\sim$2.5$\times$10$^{-3}$, in good agreement with the prediction of
astrochemical models. These models show that one of the main chemical routes to
the interstellar formation of \ch{HOCS+} is likely the protonation of OCS,
which appears to be more efficient at the oxygen end. Also, we find that high
values of cosmic-ray ionisation rates (10$^{-15}$-10$^{-14}$ s$^{-1}$) are
needed to reproduce the observed abundance of \ch{HOCS+}. In addition, we
compare the O/S ratio across different interstellar environments. G+0.693-0.027
appears as the source with the lowest O/S ratio. We find a
\ch{HOCO+}/\ch{HOCS+} ratio of $\sim$31, in accordance with other O/S molecular
pairs detected toward this region and also close to the O/S solar value
($\sim$37). This fact indicates that S is not significantly depleted within
this cloud due to the action of large-scale shocks, unlike in other sources
where S-bearing species remain trapped on icy dust grains.
