I thought our most instrumental players during that period were by far Laursen, Barry and Milner followed by Gabby. All O'Leary signings or players who came in through the youth system. Young, Carew, Bouma (O'Leary signing), Petrov, Friedel and others were important as well but the real key players were for the most part already there when O'Neil took over. He made his own additions (which were poor overall) and took away from what we had considerably by selling Davis and Cahill.
I agree with you on Sidwell and I thought Warnock was a good signing as well. I never rated Delph. Davies was a terrible signing especially considering he was bought as a replacement for Cahill. I didn't think Friedel was that much of an upgrade on Sorenson and I always rated Davis higher than Petrov.
He did get everyone working on the same page which can't be underestimated and he deserves credit for. I just wonder what might have been if things were done slightly differently.