Couple of different things you have to factor in.
First, the timing at Ohana is tricky because people tend to linger: the periodic entertainment, the way the food is served with constantly circulating servers tempting you with 'just one more piece', and the whole atmosphere contribute. I'm sure Disney tries to keep a handle on it because they don't like angry guests, but sometimes ADRs do get backed up. So sometimes you get seated right at your ADR time but sometimes you wait half an hour or more.
Second, as a prior poster noted, you can see the fireworks (and the water pageant) from much *but not all* of the restaurant. If you are going to be seated no more than 1.5 hours before the fireworks, explain at the podium when you check in that it is your first night (first trip? other special celebrations?) and ask for a table where you will be able to see out the windows. Then if you are still there when Wishes start they do dim the lights and pipe in the music, and you get a pretty good view.
Watching Wishes is always great. Watching Wishes with a bowl full of bread pudding and bananas foster sauce in front of your face is beyond great.
BUT - do make sure you get into the MK on another evening. From Ohana or from the Poly beach you don't get a great view of MM&Y, the light show on the castle about 15 minutes before Wishes. It is absolutely not to be missed.