OOC: I'm not quite sure how old Professor Oak (or even Ash) was at the time of the movie...I'm guessing around 11.
"Well, I was about 11-years-old when I met Celebi. I was in the forest, sitting down and drawing while I rested a little. Suddenly, Celebi came by and was being chased by a Scyther and a Houndoom. I was trying to help it, and we wound up going to the future. When we arrived, I was out cold, and woke up to someone carrying me. I'm afraid I wasn't very nice at first to the boy who was only trying to help me." He chuckled. "I was disoriented, and was afraid he and his two friends would hurt Celebi, who had disappeared. They helped me find Celebi, and A..." it sounded like he was going to say someone's name, but, he thought better of it, and said instead, "a man showed up and used a special ball that turned pokemon evil and automatically raised them to their highest level. He captured Celebi, and Celebi turned against us, made a big twig monster, and was decimating the forest it was supposed to protect. My new friend and I convinced Celebi to stop. Afterwards, he was week, and not even the Lake of Life healed him. That's when the other Celebi's came to help. Of course, there's more detail, but that's the gist of what happened." Professor Oak smiled. "The whole story would take much longer to tell. The Celebi I met was playful and child-like when it wasn't controlled by that twisted maniac."