HD 54662 AB is one of the three O + OB binaries known so far with orbital period longer than 1000 d, offering the opportunity to test scenarios of massive star formation and models of single stellar evolution. Here, we present a detailed study of this system based on new high-resolution spectra and data. A disentangling method is used to recover the individual spectra of the primary and secondary components, which are classified as O6.5 V(n)z and O7.5 Vz, respectively. Combining radial velocity measurements and astrometric data, a new absolute orbit with a period of 2113 ± 9 d and an eccentricity of 0.062 ± 0.008 is determined, confirming previous findings. However, absolute masses of 23.8 ± 1.1 M☉ for the primary and 20.3 ± 1.1 M☉ for the secondary are obtained, differing from previous determinations but in reasonable agreement with the spectral types of the stars. Primary and secondary components show remarkably different projected rotational velocities (160 and ≲40 <tex-math>$\rm km\, s^{-1}$</tex-math>, respectively), which is probably related to the formation process of the binary. Contrary to previously interpretations, the star with broader spectral features is the most massive object in the system. Stellar and wind parameters of both stars are derived through quantitative spectroscopic analysis of the disentangled spectra using fastwind models, and they are consistent with the current calibrations for O-type stars. Evolutionary masses and ages are also computed with the bonnsai tool. Ages below 2.5 Ma are obtained, in agreement with the youth expected from their Vz nature.