Although there are indications of the existence of people in this municipality from the Palaeolithic (an Acheulean biface was located in the Silvarrei area ), the first major traces of Outeiro de Rei ‘s historical past date back to the Megalithic period (4000-2000 BC ). The abundance of “mámoas” (prehistoric tomb from the Megalithic) located in its term, among which the “Medorra da Pedra Dereita” in Martul and grave goods found in these funerary monuments (preserved today in museums and private collections) and the rock taxed or petroglyphs scattered its mountains, reveal the presence during this stage of a fairly large and active population.

From then until now with the existing literature and archaeological references, it is logical to think that this place was inhabited continuously. First, by the “castreña” population (800 B.C.-100 A.D.), with several “castros” (walled villages) listed in this municipality, all unexcavated, and among those is the Castro of Candai located within the boundaries of the parishes of Aspai and Candai.

Later, the Romans (60 BC-410 AD), which are supposed to arrive here shortly after founding the city of Lugo (25-20 BC), baptized this place with the name of Octodorum (where the Roman road XX Braga-Astorga passed through), according to most historians and classical sources consulted. But the medieval period (s. VIII-XV) is undoubtedly when this town experienced its greatest splendor, after the Arab invasion and later conquest of the territories of the province of Lugo by bishop Odoario, territories among which were the parishes of Silvarrei, Cela and Parada.