Oskar is a Russian child of eleven years in age. He is strong, brave, and proud. His father has been killed for disagreeing with the state. Oskar becomes fast friends with Ida and Ping when he goes to stay with them at Green Knowe for the summer. He is strong-headed and adventurous, and becomes the size of a mouse while on Mouse Island. Along with Ida and Ping, Oskar brings Dr. Biggin to the circus to see Terak the giant, but Dr. Biggin is unimpressed, believing Terak to be a fake.
