Tutorial Step 20: Set Up Project Syncing
When multiple users will be contributing to and editing an OmniPlan project, you can easily keep everyone on the same page by storing the project in a server repository. First, set up a server account in the Accounts section of Preferences, found in the OmniPlan menu. 
Select an account type from the list that includes CalDAV, Google, Omni Sync Server, and WebDAV. The Omni Sync Server option is recommended for users seeking a free and robust syncing solution, while a self-hosted WebDAV server may be preferred for confidential projects where data privacy is the highest priority.
To sync via Omni Sync Server, enter your account information or click Create Account to create a new account online. Note that in OmniPlan the user name field only requires the part of your email address that precedes the "@" symbol, and the directory field can be left blank. You'll get feedback at every step of the process as your account data is validated.
Next you'll want to visit the Configure Publishing & Subscriptions screen, accessible via the Projects menu. Click the plus sign at the bottom of the Publish & Update Actions window to choose the type of syncing or subscription you'd like to set up for this project. Choosing Sync entire plan with OmniPlan server repository will make the project accessible to everyone else with access to the account you've just set up in preferences, and updates to the project file will be synced for all users.
Note that because OmniPlan relies partially on your system time to determine the scheduling of synced project updates, making changes to the Apple system clock when working on a project with publishing or subscription enabled will result in an error and should be avoided.