O. Tidemann is station master for Sesam Stasjon, a kind but somewhat blustery grandfatherly-type who generally manages to keep the station running smoothly, even if he doesn't always get the peace and quiet he wants.

His full name is a mystery; not even the other characters know what the "O" stands for (at one point Alfa asked him if the "O" was short for Otto, but he denied that). However, it does seem that it's a family tradition to name the children alphabetically: Tidemann's father was named N. Tidemann, his grandfather M. Tidemann, his great-grandfather L. Tidemann, and so on. All of them were also station masters on Sesam Stasjon, so it seems like the job also runs in the family.