Last April we went and we had booked O'hana. About a week before our trip we got a call from a friend who lives in FL and wanted to come spend that same day in the parks with us so I had to cancel our reservation. I tried to get a new one but they were full.
On 4/27 we left Magic Kingdom around 7:00 and thought we'd just go over and check out the Poly. We took the monorail and when we got there, totally on a whim, we went up to O'hana and asked about possibly having dinner. The gal gave us a buzzer and told us it would be a wait but that they could get us in. I waited and had a drink whilie the kids and dad walked around and checked out the hotel and then about 30-45 minutes later (around 8:15pm I think...) we had a table. We had a nice relaxing dinner and as we were finishing the fireworks started at MK. I didn't know you could see them from O'hana but not only could we see them, we had the best view in the whole resturant. It was one of the highlights of that trip!
I consider it one of our very lucky WDW moments but those do happen all the time. I'd keep trying and don't hesitate to try to walk up. If you go at a really off time you may just get lucky. If not, Kona is also there and others have reported being able to walk up there.
You could also post on the cancel/trade reservations board. I did that for the Le Cellier dinner that my son really wanted and in 2 days I got a PM from someone offering me thier dinner reservations that they were cancelling. I was really surprised!