The school experienced numerous changes during Osmond's time there.
"We're in a new school, we've had huge roll growth and our approach to teaching and learning has been personalised to our students," Osmond said.
"Seeing the successes of our children and seeing them grow over the period of time that I've been here has been a real buzz for me."
Leaving with Osmond is his wife Emma, who is a teacher at St George's School. Their time will be celebrated at a morning tea following chapel service on Sunday, June 24.
A highlight for them both was being at the school while their children attended.
"I was incredibly lucky in my time to have my two children attend St George's," Osmond said.
"They've left now, but to see them grow, to see them flourish and develop passions and a love for learning was just great."
Osmond said he was looking forward to saying goodbye to everyone at the morning tea, despite the fact that it would be emotional.
"I would hope that people think I have done an okay job.
"Five years has gone in a flash, that's one of the scariest things when you're so busy in school, if you don't take time to reflect, if you don't move now, another five years will be gone in a blink too.
"It's about knowing when the time is right to go and that time for me is now."