The Ontario Regiment Museum is a strong niche stop, especially if you care about military vehicles. Its collection includes tanks, armoured cars, and regimental material, with the airport setting giving it room for machinery that would overwhelm a normal downtown museum.
It is more specialized than Parkwood or the car museum, but it rounds out Oshawa's heavy-metal side nicely: not just cars and assembly lines, but engines, armour, and 20th-century Canadian service history.