Oswald's Diary is the diary of the former Berserker chief, Oswald the Agreeable. When he went missing and his son, Dagur, and daughter, Heather, started to look for him, they used the book to find clues to where he could have been.
This is a diary belonging to Oswald the Agreeable which later came into the possession of Dagur and Heather, who used it in their search for him. However, it only leads them to dead ends. To prevent Heather from going into a suicide mission, Dagur secretly takes the diary from her. He then goes to Dragon's Edge and shows it to Hiccup, asking him for help. Hiccup then finds in the diary the sketch of a Sentinel, a dragon only found on Vanaheim.
Before they leave, Fishlegs looks through the diary too, showing the others Oswald's sketches of the island.
- It is strange that the diary came into the possession of Dagur and Heather, considering that after Oswald disappeared, he died on Vanaheim.