OH– is considered a base. This is because it is a hydroxide ion, and hydroxide ions can accept protons (H+) in chemical reactions. In the Brønsted-Lowry acid-base theory, bases are defined as substances that can accept protons.
When OH– accepts a proton (H+), it forms water (H2O), which is the conjugate acid of the hydroxide ion. Therefore, the conjugate acid of OH– is H2O.
To summarize, OH– is a base, and its conjugate acid is H2O.