The phrase "say so" is often used to indicate that someone has made a statement or given their opinion. However, there are several synonyms that can be used in its place. These include "assert," "affirm," "declare," "state," "proclaim," "attest," "avow," and "pronounce." Each of these words carries slightly different connotations and nuances, which can help to convey a specific attitude or tone in a statement. Choosing the right synonym for "say so" can add variety and clarity to one's writing or speaking, allowing for more effective communication.