The Municipality of OAS has one big(1) land mass configuration with a total land area of 291.7335 Square Kilometers and accounts for about 11.43% of the total land area of Albay Province. The Urban Barangays has a total land area of 10.7097 Square Kilometers while the Rural Barangays has a total land area of 281.0238 Square Kilometers, Barangays Mayao and Centro-Poblacion have the largest and smallest land areas among the Urban Barangays while Barangays Cagmanaba and Tapel have the largestand smallest land areas among the Rural Barangays. A total of 53 Barangays comprises the Municipality of OAS of which 12 Barangays are considered Urban while 41 Barangays are considered as Rural.

The Municipality of OAS is traversed by numerous rivers and creeks not to mention its coastlines along Burias Pass which serves as natural drainage system. A notable river system is the Quinale River which is trans-municial in scope. However, due to the inherent land formation of the municipality lans areas which are level to undulating in-between steep and mountainous land areas are exposed to flooding hazard including those land areas along coastal barangays.

The geologic structure for the Municipality of Oas is composed of Sedimentary Rocks. The Sedimentary Rocks are characterized by the following a.)Widespread Pyroclastic Agglomerates, Volcanic Breccia, Tuff, Pumice and Volcanic Debris; b.)Terraced Gravel and Alluvial Deposits; and c.) Widely Exposed and Extensive Conglomerates Sand Stone and Calcoreous Siltstone. The Igneous and Metamorphic Rocks are characterized by a.) Andesitic & Dositic Volcanic Rocks; and b.) Diorific Intrusive Rocks