  The quantum anomalous Hall effect resulting from the in-plane magnetization
in the OsCl$_3$ monolayer is shown to exhibit different electronic topological
phases determined by the crystal symmetries and magnetism. In this Chern
insulator, the Os-atoms form a two dimensional planar honeycomb structure with
an easy-plane ferromagnetic configuration and the required non-adiabatic paths
to tune the topology of electronic structure exist for specific magnetic
orientations based on mirror symmetries of the system. Using density functional
theory (DFT) calculations, these tunable phases are identified by changing the
orientation of the magnetic moments. We argue that in contrast to the buckled
system, here the Cl-ligands bring non-trivial topology into the system by
breaking the in-plane mirror symmetry. The interplay between the magnetic
anisotropy and electronic band-topology changes the Chern number and hence the
topological phases. Our DFT study is corroborated with comprehensive analysis
of relevant symmetries as well as a detailed explanation of topological phase
transitions using a generic tight binding model.
