The O1 visa is one document required by the US government from aliens who intend to work in the US as non-citizens. You can expect to stay in the US a minimum of three years if you already have been granted an O1 visa. However, your stay could be readily extended if you still have work duties with your US-based employer. Your employer will have to provide proof that your services are still needed; hence an O1 visa renewal would be required.
Your employer can petition that your stay be extended on a per-year basis. If the US government feels that an O1 visa renewal is required, you can expect to be able to stay another three years because each petition by your employer can be considered a new case.
To qualify for an O1 visa renewal, you have to provide the following: the Form Petition for Non Immigrant Worker (also called Form I-129); your Arrival/Departure Record (also called Form I-94); and a letter from your employer clearly giving the reason for the extension being sought. Bear in mind that once the USCIS has denied the petition, it cannot be appealed.
Your employer can cite a single event or a series of events as basis for the renewal of your O1 status. Aside from main events like a business conference or additional work, the employer may also cite incidental events like a vacation as basis for the renewal. Your contract with your US-based employer may also be considered a justified event.