OP: Disney restaurants do not hold back any tables for walk-ups, and Ohana is now one of the places that charges you $10 per person if you are a no-show or don't cancel at least 1 day before your ADR. Check the ADR system one last time the night before you'd like to eat there but if there are no tables available, then there really are no tables available and trying a walk-up is just going to waste your time.
The food is good and the theming is great; a very tropical vibe. It is a single TS credit. When you are seated you are given bread and then a green salad and 2 appetizers (chicken wings and pot stickers) are brought to your table, plus dipping sauces. Servers are constantly circulating with skewers of beef, pork, chicken, and shrimp that are being cooked over the open fire in the center of the restaurant. Whenever you are ready, get some from a passing server and enjoy. When you can't possibly eat any more they bring you what I think is one of the best desserts in WDW: a big bowl of bread pudding topped with ice cream and a pitcher of warm bananas Foster sauce to pour over it. It may sound a bit dorky but trust me: it's wonderful. Periodically through the night an entertainer comes out; sings a bit and gets the kids up to roll coconuts or do a limbo contest.
If you eat very early it's convenient to hop on the monorail to go back to MK; if you eat during the evening it's popular to go down to the Poly beach afterwards and watch the Electric Water Pageant and then Wishes. Some tables in the restaurant actually get at least a partial view of Wishes, so they dim the lights and pipe in the music, but you can't count on being able to see from where you sit and unlike Cali Grill there is no outdoors observation area.