What is particularly pleasing about the forest on Önsholm island is the large oak trees and the extensive surface area. This island is home to diverse forests with plenty of decayed trees, from rugged rocky pine woods to lush oak and broad-leaved forests. The cliff on the eastern end of Önsholm has a lovely view towards Harvaluoto island in Piikkiö.

A memorial corner for the Finnish Natural Heritage Foundation’s donor Jyrki Arnikari (1947–2022) is located on the island. The donation included in Arnikari’s will had a significant impact on the protection of Önsholm forest.