Oliver found Silver after it had wandered away from its parent. Despite the fact that he never captured it in a Poké Ball, Oliver claimed it as his own. He played with Silver all day long and they both enjoyed it.
However, Team Rocket was also watching. Dr. Namba assigned Butch and Cassidy to capture Silver in order to lure the parent Lugia in. This was part of Namba's plan to attach his invention to Lugia, making it extremely angry and extremely powerful.
Ash, his friends, and Ritchie all teamed up to free Silver and its parent. After freeing Silver, Ash and Ritchie tried to convince the parent Lugia that not all humans are bad. Their convincing worked, since Lugia used Safeguard to protect Ash and the others from falling debris.
Oliver agreed that Silver should be released to its parent. Although he was saddened, he knew that it was for the best.
