Results for OMC-2 and OMC-3. Panel a: depletion map (ƒ_D) obtained from the ratio of the expected abundance of C¹⁸O (X₀(C¹⁸O)) in Orion and the measured abundance (N_tot(C¹⁸O)/N(H₂)). The black contours represent I(N₂H⁺) > 4 K km s⁻¹ (signal-to-noise ratio of 10; see Fig. A.1, panel b). Panel b: abundance of o-H₂D⁺ (X(o-H₂D⁺)) inferred from the C¹⁸O depletion factor using the correlation from Sabatini et al. (2020). Panel c: deuteration fraction, determined as R_D = N(DCO⁺)/N(HCO⁺), in OMC-2 and OMC-3 (see text for its determination). Panel d: ${\zeta }_{{\text{H}}_2}^{\text{ion}}$ determined with Eq. (1). The white star represents a B-type star within the selected region boundaries. The dotted red circle is a candidate expanding CO-shell (Feddersen et al. 2018) possibly connected to the star.