I'm rather indifferent to OPC unless the kid is a notable horror (rare), but I do find myself inwardly gnashing my teeth at other parents.
In particular:
Those who cannot hold a conversation without cutting it off every three minutes so we can listen to their child interrupt and talk meaningless bollocks which we are supposed to be charmed and delighted by. Those parents who won't say, "Off you go and play xxxx, I am talking to (other adult) just now."
Those who glibly and fondly use the terms 'spirited', 'boisterous' and 'rough and tumble' to describe what is essentially their kid thumping lumps out of yours.
Those who simply cannot bear to see their child upset so will try to engineer every scenario to suit their demands, whether it be wanting another child's toy or where to eat for lunch. When children are used to being put on a pedestal this way they turn into obnoxious little gits through no fault of their own but the feebleness of their parents.
I spent a weekend camping with one such father and son combo after which we never spoke again. The lad wanted every possession our son had, whether it was his chair, his torch, his Star Wars water bottle and our son, who was younger than the other kid, was repeatedly requested to hand them over.
The stupid man was most put out when I eventually told him to get a fucking grip and leave our son's stuff alone.
In all of those scenarios the kids display some pretty unappealing behaviour but to my mind it's the parents that cause it, not the kids.