While Onewhisker was pretty kick-butt, I absolutely hate him as Onestar. I understand that he was trying to distance himself from ThunderClan to prove that WindClan didn't need them to survive, but his character did a complete 180 practically overnight, and he went from being one of Firestar's closest friends to being a complete stick in the mud.
I also loved Onewhisker and I'm still really confused by how much he changed when he became Onestar. I mean, I understand that he would become more of a stick in the mud to some extent, but it always struck me as just bizarre how much he changed and how quickly. Onestar didn't read like Onewhisker with an attitude adjustment, he read like a completely different character in every way. I don't know what the purpose of that is from a narrative standpoint? It's just not believable at all.
It's been a while since I read the later parts of the second series, but I always understood it as less an actual change in his personality and more a response to WindClan thinking he was too friendly with ThunderClan/too soft of a leader. Basically, he was severely overcompensating to try and prove that he was a strong leader.
I still agree that it wasn't executed very well, though. And it doesn't explain why he doesn't mellow out later on.