In order to derive precise vibrational constants of diatomic molecules the usual procedure needs the origins of rotationally analysed bands. However, the knowledge of these origins is a very hard task, and most commonly only a few bands are known with such a detail. For that reason, we have chosen a procedure suggested by Jenkins and McKellar, that we followed with corresponding branches of the 1-0, 0-0 and 0-1 bands of the B-X system of LaO. For consistency, only measurements reported by Åkerlind were used throughout.