In this work, optical, including photoluminescence and photosensitivity, characteristics of micrometer-sized flexible n (p)–InSe/In₂O₃ heterojunctions, obtained by heat treatment of single-crystalline InSe plates doped with (0.5 at.%) Cd (Sn), in a water-vapor-and oxygen-enriched atmosphere, are investigated. The Raman spectrum of In₂O₃ layers on an InSe:Sn substrate, in the wavelength range of 105–700 cm⁻¹, contains the vibration band characteristic of the cubic (bcc-In₂O₃) phase. As revealed by EDX spectra, the In₂O₃ layer, ~2 µm thick, formed on InSe:Cd contains an ~18% excess of atomic oxygen. The absorption edge of InSe:Sn (Cd)/In₂O₃ structures was studied by ultraviolet reflectance spectroscopy and found to be 3.57 eV and ~3.67 eV for InSe:Cd and InSe:Sn substrates, respectively. By photoluminescence analysis, the influence of doping impurities on the emission bands of In₂O₃:Sn (Cd) was revealed and the energies of dopant-induced and oxygen-induced levels created by diffusion into the InSe layer from the InSe/In₂O₃ interface were determined. The n (p)–InSe/In₂O₃ structures display a significantly wide spectral range of photosensitivity (1.2–4.0 eV), from ultraviolet to near infrared. The influence of Cd and Sn concentrations on the photosensitivity and recombination of nonequilibrium charge carriers in n (p)–InSe layers from the heterojunction interface was also studied. The as-obtained nanosized InSe/In₂O₃ structures are suitable for optoelectronic applications.