You may be lucky, or you may be turned away. If you are going without a reservation, get there EARLY. They start lining up at 4 or even before, mostly people who don't already have a reservation and are hoping to get lucky. 'Ohana will not start checking people in until 4:45. It will all depend on what is available and when. They may tell you that you would need to wait a couple of hours for a table, if you are up for that. But they may also tell you that they will not be able to admit anyone without a reservation.
Sometimes, they will have a CM walk the line before they open check-in, asking if the persons in line have reservations, so they can state there is no space in the restaurant and clear the line of those without reservations before they open. They don't do this all the time though.
'Ohana dinner, at least anytime before 8:30 PM or so, has now easily supplanted Le Cellier as the hottest meal ticket at WDW.