The petitioner for your O-1 Visa petition can be a variety of people and is generally referred to as the U.S. Agent. Self-petitioners are not allowed for the O-1 Visa.
The U.S. Agent is someone who acts on your behalf and is a facilitator for the petition. The Agent can be a variety of people whether it is your employer or not. If you have multiple employers, it can be helpful to have the Agent be a third-party who is authorized by your employer to act in their place and be representative of you and your employer(s). In this case, the Agent could be an attorney, the manager of the extraordinary individual, or a presenter or organizer.
The U.S. Agent has several responsibilities when it comes to the petition. For instance, they are meant to clarify the terms and conditions of the employment and provide any required documentation like the contract(s). They are also responsible for the itineraries for all the events the employee will act in, and provide the full information regarding where these events will take place.