Østby was born in Modum, but grew up in Bærum, and was a member of Bærum KK. He was a new name in Norwegian and international kayaking after 1945, as most of the pre-war organized sporting activity had been put on ice during World War II. Already in 1948, before the Summer Olympics that year, Østby and his teammate Ivar Mathisen were considered as having the highest medal hopes in the Norwegian canoeing squad. Østby and Mathisen proceeded to win the silver medal in the K-2 10000 m event and finished fourth in the K-2 1000 m. At the 1950 World Championships he won another silver medal in the K-2 1000 m event. Two years later he finished fifth in both the K-2 1000 m and the K-2 10000 m events at the 1952 Summer Olympics. Ivar Mathisen remained his partner in the K-2 canoe during these years.