Brownlow medallist Mitchell went to Collingwood and O'Meara has joined Fremantle, with veteran forward Jack Gunston also released to join the Brisbane Lions.
The trade period moves have been queried by some pundits but Mitchell says they're strategic long-term decisions.
"(The trio) were brilliant contributors to our club. Not just great footballers, but also great people," Mitchell wrote in a letter to Hawks members.
"We wish them well. These decisions also give them a fresh opportunity to continue their football journey.
"A lot of people may say, well you are losing some of your older players and experience.
"Well, my response to that would be: your age doesn't define your ability to lead.
"We have known for some time we would need to find our next wave of future leaders at our club and that is why over the past 12 months we have spent a significant amount of our focus on this."
Mitchell won eight games this year, finishing 13th in his first as Hawks coach, but said he had a firm plan.
He described the just-completed trade period as "the next step in a longer plan to develop our list and return to premiership contention".
"To be honest, we don't get there if we follow the path of least resistance," he wrote.
"This is about building our next phase of talent.
"I have no interest (and I know the players feel the same), to just sneak into the finals. Internally we are working on building premiership standards because only then can we become a premiership team.
"Premiership teams are not built overnight, we want to be in a position where we are competing to win it all when our time comes - whenever that may be."
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