Unfortunately, there is no surviving direct account of Olmec beliefs, but their artwork provides clues about their life and religion. 

Olmec King

Surviving art, like this relief of a king or chief found in La Venta, help provide clues about how Olmec society functioned.

There were eight different Olmec deities, each with its own distinct characteristics. For example, the Bird Monster was depicted as a harpy eagle associated with rulership. The Olmec Dragon was shown with flame eyebrows, a bulbous nose, and forked tongue. These gods were believed to provide the rulers a mandate to lead. Deities often represented a natural element and included:

• The Maize deity
• The Rain Spirit or Were-Jaguar
• The Fish or Shark Monster

Above is the Olmec dragon carved in stone from Latin American Studies http://www.latinamericanstudies.org/olmec-dragon.htm

Religious activities regarding these deities probably included the elite rulers, shamans, and possibly a priest class making offerings at religious sites in La Venta and San Lorenzo.