As others have said, you will see a variety of dress at 'Ohana.
Many people come straight from the parks for dinner at 'Ohana, & so they'll be wearing whatever they wore to the parks. Others will be slightly "dressier" - sundresses, capris, dress shorts, etc.
For resort restaurants, we do tend to go back to our room to change clothes & freshen up before dinner. We also go back to our room & change/freshen up for CRT as well.
For 'Ohana, I try to do sort of a "tropical" look... And we're one of those silly matchy-matchy families too... although, now that the kids are older, I can't say we "match". Instead, I have to plan "coordinating" outifts. There's a difference.
Anyway, last year, I wore capri pants & a dressier top. DD wore an Hawaiian-print skirt w/ a matching top. DSs wore coordinating Hawaiian-plaid shorts & shirts from Gymboree. DH wore dress shorts w/ a collared top.
We fit in fine - there were some families there that weren't as dressy, & there were some that were dressed about the same as we. I saw many women in sundresses & maxi dresses. In prior visits, I've worn a skirt, a sundress, capris, & shorts.