Foundation of the municipality can be traced to the early Spanish Colonization of the Bicol Peninsula. In 1605, Father Baltazar de los Reyes converted 12 leading natives of the area to Christianity in one single day, forming the foundation of the community now known as Oas.
There are three stories that tell the origin of the name of Oas:
- The first version mentions the existence of numerous lagoons or pools of water abounding the place which prompted early colonizers to give it the name of "Oasis". The natives soon called the place by this name and later abbreviated it to "Oas".
- A second version relates to the existence of a dam across the narrowest portion of a local river. This dam solely irrigates the vast fields of the place including those of nearby town of Libon and results in a good harvest. People are wary of any cracks or leaks on the dam walls during months of heavy rains. A crier would shot "nawaswas" giving the call to the people for immediate action in groups. From then on, the natives coined this name to the place and later shortened it to present from in times of this kind emergency.
- A third version of how the municipality derived its name gives an account of early Spanish Colonozers reaching this particular section of the Bicol Peninsula. The Spaniards, inquired the name of the place from the about 600 natives found living there, "Como se llama este sitio?" with gesture of their hands. The natives mistakenly thought the question to be, "Onan kading lugar kadi, maiwas?" (What place is this, it's very big?) in their native dialect. In response, the natives answered, "Si, señores. Labi nikading iwas. Labi nikading iwas." (Yes, sirs, this is grand and spacious). From then on the early Spanish colonizers adopted in their official census the existence of "a rich fertile valley with verdant fields of grain" which is a little town of Oas in Bicol.