Scheduled time. Unlike a structured MA, you'll be much on your own, so imposing a timetabled structure and agreeing targeted deadlines with a supervisor is essential.
You didn't say whether its an OU PhD or with somewhere else? Doing a P/t one in an institution geared to full time can mean you miss out though if you make contacts with other PhD students regularly then that can help (though the different time scales may mean less in common as time goes on)