  We report the results of our follow-up campaign for the neutron star - black
hole (NSBH) merger GW200115 detected during the O3 run of the Advanced LIGO and
Advanced Virgo detectors. We obtained wide-field observations with the
Deca-Degree Optical Transient Imager (DDOTI) covering ~20% of the total
probability area down to a limiting magnitude of $w$=20.5 AB at ~23 h after the
merger. Our search for counterparts returns a single candidate (AT2020aeo),
likely not associate to the merger. In total, only 25 sources of interest were
identified by the community and later discarded as unrelated to the GW event.
We compare our upper limits with the emission predicted by state-of-the-art
kilonova simulations and disfavor high mass ejecta (>0.1$M_{\odot}$),
indicating that the spin of the system is not particularly high. By combining
our optical limits with gamma-ray constraints from $Swift$ and $Fermi$, we
disfavor the presence of a standard short duration burst for viewing angles
$\lesssim$15 deg from the jet axis. Our conclusions are however limited by the
large localization region of this GW event, and accurate prompt positions
remain crucial to improving the efficiency of follow-up efforts.
