The first high-resolution (R˜ 50 000) optical spectrum of the B-type star, LS III +52°24, identified as the optical counterpart of the hot post-asymptotic giant branch (post-AGB) candidate IRAS 22023+5249 (I22023) is presented. We report the detailed identifications of the observed absorption and emission features in the full wavelength range (4290-9015 Å) as well as the atmospheric parameters and photospheric abundances (under the local thermodynamic equilibrium approximation) for the first time. The nebular parameters (Te, Ne) are also derived. We estimate Teff= 24 000 K, log g= 3.0 and ξt= 7 km s-1, and the derived abundances indicate a slightly metal-deficient evolved star with C/O < 1. The observed P-Cygni profiles of hydrogen and helium clearly indicate ongoing post-AGB mass-loss. The presence of [N II] and [S II] lines and the non-detection of [O III] indicate that photoionization has just started. The observed spectral features, large heliocentric radial velocity, atmospheric parameters and chemical composition indicate that I22023 is an evolved post-AGB star belonging to the old disc population. The derived nebular parameters (Te= 7000 K, Ne= 1.2 × 104 cm-3) also suggest that I22023 may be evolving into a compact, young low-excitation planetary nebula. Our optical spectroscopic analysis together with the recent Spitzer detection of double-dust chemistry (the simultaneous presence of carbonaceous molecules and amorphous silicates) in I22023 and other B-type post-AGB candidates may point to a binary system with a dusty disc as the stellar origin common to the hot post-AGB stars with O-rich central stars.