BiCuSeO is a promising thermoelectric material, but its applications are hindered by low carrier mobility. We use first-principles calculations to analyse electron–phonon scattering mechanisms and evaluate their contributions to the thermoelectric figure of merit ZT. The combined scattering of carriers by polar optical (PO) and longitudinal acoustic (LA) phonons yields an intrinsic hole mobility of 32 cm² V⁻¹ s⁻¹ at room temperature and a temperature power law of T^−1.5 between 100–875 K, which agree well with experiments. We demonstrate that electron–phonon scattering in the Cu–Se layer dominates at low T (< 500 K), while contributions from the Bi–O layer become increasingly significant at higher T. At room temperature, ZT is calculated to be 0.48 and can be improved by 30% through weakening PO phonon scattering in the Cu–Se layer. This finding agrees with the experimental observation that weakening the electron–phonon interaction by Te substitution in the Cu–Se layer improves mobility and ZT. At high T, the figure of merit is improved by weakening the electron–PO phonon scattering in the Bi–O layer instead. The theoretical ZT limit of BiCuSeO is calculated to be 2.5 at 875 K.