  Hexagonal manganese oxides RMnO$_3$ show intriguing topological
ferroelectric-domain walls with variable conductivity, leading to domain wall
engineering. Despite the numerous experimental studies on the polar nanoscale
structures, controlling ferroelectric domains has not been sufficiently
investigated. Here, we reveal the unprecedented coexistence of polar-nonpolar
nanoscale domains that can be formed by substituting Ti ions in HoMnO$_3$.
Unusual polar nanoscale domains are embedded in nonpolar domains with different
crystallographic symmetry. This polar-nonpolar coexisting structure is
naturally assembled by adjusting the lattice length during a solid-state
reaction process. Furthermore, the comprehensive study reveals that the
reversible microstructural change with a nonpolar-polar transition is strongly
correlated with the oxygen storage properties in Ho(Mn, Ti)O$_{3+\delta}$. The
present results provide important insight into the nanoscale polar-nonpolar
domain coexistence in functional rare-earth manganese oxides, RMnO$_3$.
