OmniPlan (and the Omni Sync Server) don't currently have any ideas about access control inside a given account. OmniPlan does support setting up multiple accounts, though, and the "which account do I sync to" setting is per-document. You'll want to set up different accounts for different projects.
I'm reasonably sure there's a feature request open on the "let this person see branch X, but not the rest of the plan", but that's also not supported yet.
If that feature would help your workflow, email the
support ninjas and we can attach you to that item in our development database - all other things being equal, more popular requests get priority when we're planning our work. Thanks!