From Bulbapedia, the community-driven Pokémon encyclopedia.
Ogi Isle (Japanese: 沖島 Oki Isle) is the northernmost isle in the Whirl Islands archipelago. It is an anime-exclusive area in Johto, located off the northwestern tip of Silver Rock Isle, almost near Olivine City. The two islands are connected to each other via an underwater tunnel.
In Hi Ho Silver... Away!, Ash and his friends met silver rock craftsman Jenaro, who told them about a legend revolving around the mysterious Silver Wing. According to him, many generations ago, a ship sunk during a storm off the coast of Ogi Isle, but a Lugia saved the crew. After the skies cleared, a Silver Wing fell from the sky, and since then, people have revered the Silver Wing as a symbol of good fortune. Following Jenaro's recommendation, the trio later went to Ogi Isle.
While on Ogi Isle, Ash and his friends were reunited with Luka and Ritchie in The Mystery is History. They also met Oliver and his friend, a young Lugia nicknamed Silver. Silver's parent later appeared in a rage, but Ash and the others reassured Lugia of their good intentions and this managed to calm it down. Following Dr. Namba's orders, Butch and Cassidy later arrived on the scene and kidnapped Silver.
In A Parent Trapped!, the group worked to rescue Silver from Team Rocket, but the parent Lugia was also captured. Later in A Promise is a Promise, Ash and his friends were able to rescue the two Lugia and Dr. Namba's underwater base was destroyed during the clash. The child and parent Lugia both said farewell to Oliver and the others before they swam out to sea.