The first document attesting the existence of the village dates back to the year 1194, when the village of Olmo is mentioned in an act concerning an investiture. Recent studies indicate, however, that the first settlements were before the XI century. The toponym certainly derives from Ulmus, that is the elm tree plant, particularly present in the territory at that time.
Situated on the territory of the Holy Roman Empire, it was given by Charlemagne to the monks of the Abbey of Tours. Subsequent exchanges and investitures saw the areas pass under the command of the Della Torre family.
They also dominated the nearby Valsassina, seat of the feud, which consequently became the centre of reference for the town of Olmo and most of the valley. Subsequently the command passed to the Visconti family, who granted special statutes, tax relief and great autonomy.
The arrival of the Venetian domination confirmed the privileges previously granted and favoured commercial traffic, already flourishing thanks to the presence of the Via Mercatorum, the artery of traffic with the nearby Val Seriana. The Serenissima built a new road, the Via Priula, which directly connected the capital Bergamo with the canton of Graubünden, in what is now Valtellina. In this regard, in the locality of Malpasso, there was a customs office that was used by transporters coming down from the San Marco Pass, which was in operation until the end of the 19th century.
In these centuries a society called Società degli Originari was created in the village, in which all the families who had lived in the village for the longest time were grouped together. This in order to protect their interests towards the "strangers", i.e. those who, coming from other areas, purchased land on municipal land. With this association, these families wanted to maintain privileges on land passed on to others, such as timber harvesting, cutting hay and grazing animals.
In 1863, just after the unification of Italy, the village took the definitive name of Olmo al Brembo.