When does the office open/how do you check in? We're trying to arrive right when it opens so we can get a walk-in site.

No office... you self check in at board... so note your campsite number...

There is no office, you just show up whenever and fill out your envelope with your money and put it in the drop box. I think you can reserve online, otherwise it's just first come first serve. There's a big board at the main enterance/day use parking area that shows what isn't reserved already.

Check with Olympic National Park online