Tap on your project to view its details. The first set of 3 buttons controls whether your project is sequential, parallel, or a collection of single actions.
We changed the default behavior of OmniFocus 2 for iPhone so that (if I'm remembering right!) either a project or a context is sufficient to move an action out of the inbox. On the Mac there's a setting to control that behavior (and that setting will sync to the iPhone), but we haven't added it to the iPhone yet.