"You start in September and, before you know it, it's Thanksgiving. Then it's February and the year's almost over. I mean it just flies by," said O'Brien.
On Thursday, McCracken taught his class of 8th graders for the last time before they head off to high school.
"The nice thing about teaching is that you are constantly learning. I learn new things every year, try to make new connections to make things simpler for my students. So I'm always learning and I love history," said O'Brien.
"I think Mr. O'Brien is one of the best teachers I ever had," said Clark Fox, student. "The experience with him has been inspiring."
"He encourages all his students to really take part in history to learn it and understand it," said Vanesse Cadvillo, student.
In September, a new wing on the school will be dedicated to O'Brien.
"He really is a walking history book of the district-- been here forever. He can tell you what we've done. What's worked? What hasn't worked? What we should try again. He knows all the teachers from the past. He knows everything about the district. It's amazing," said Kate Donegan, district superintendant.
"Some of them write for newspapers. Some of them are judges. Some of them are doctors. Some of them are selling automobiles. Some of them are working in department stores. I meet them everywhere… and it's great," said O'Brien.