Opie generally doesn't care much for wearing a suit. He'll wear one to church or when traveling, or when he thinks his father is about to be married, whether he really is or not.
And then, there is the time that Briscoe Darling buys Opie a new suit when he is "betrothed" to Andelina Darling ("The Darling Baby," Episode #139). Opie sure wishes he could disappear that time.