Best time of day for big waves in Oahu?

North Shore in January. Waimea is the place with big waves close in. You'll find parking impossible. Up the road toward Pupukea/Sunset Beach area, you'll see some big waves gettig surfed. Last time I was there, they had 10-12 foot waves but they were pretty well offshore. Take your binoculars, beach chair, some shady hat or umbrella, and a cooler and you'll be set for the day.

But as it was pointed out previously, you're not going to see this all the time. Check the surf reports and forecasts. Here are a couple from a whole bunch that are out there:

http://www.hawaiiweathertoday.com/oahu.php

http://www.prh.noaa.gov/hnl/pages/SRF.php

As others have pointed out, there is no best time of day as far as when the big waves will happen. But as far as being on the beach for viewing, that is another matter. If it is just another day from a wave perspective, you could go pretty much anytime. But if it is truly one of those "big wave" days, and there are just a few of those per season, then the earlier you go the better. You will need to be on the north shore early in the morning to snag a parking place.

There is no best time as the size of a wave is determined by the weather not the time. Surfing at night is not a good idea though. Best you can do is to ask at any of the surfing centres that dot the North Shore.