I would plan to arrive at the Polynesian Resort 45-60 minutes before your ADR... to be on the safe side,. Sounds like a long time but you never know when transportation will be delayed... and the check-in process to get a pager can (not always) be a long wait. And if there early, you can find things to do to entertain yourself while waiting for your page to be seated.
If you take the boat, it goes to the Grand Floridian 1st, then the Polynesian, then back to MK. The monorail drops you off right at the ceremonial house (main building) where the restaurant is.
Been to Ohana twice - 2009 and 2010. We didn't have a great experience at Ohana this most recent trip... it possibly could take a lot of time from MK, but if you don't mind, then no big deal. It opens at 5pm, so if you really want to be seated as timely as possible... I would arrive well before then. I'd get to the restaurant to check in by 4:45pm, there may already be a line just to check in and get a pager. Plan to wait anywhere from 0 to 60 minutes to get seated, based on my experience and what I've read in the reviews. While waiting, grab yourself a drink and check out the gift shop, take some photos, etc.. I'd really like to know how far I could go with one of the pages and still have it work (could I walk to the beach?)
Here's a few tips to make your experience better... if you don't already know.
Go to the 1st floor resort registration desk. Ask for Leis - they have fabric leis (no charge). Ask for a resort activities schedule and when the welcome the night ceremony is. On most nights (I think nights when they have the Spirit of Aloha show), performers from the show do a short (10 min?) ceremony just outside the main building around 6pm - drumming, fire juggling, lighting the tiki torches.
You can view wishes fireworks from the beach by the Polynesian's volcano pool... and the electric lighted boat parade passes by a few minutes after 9pm.
While on the monorail, if you haven't been to the Grand Floridian, I'd visit their lobby. In the late afternoon and evening, there's a piano player and small orchestra that play. The lobby is beautiful and some neat floor tile work with characters.
Have a great Trip!