An OST file is not the same as a PST file. It's a local copy of your mailbox, but in a mangled, dysfunctional format.
PST files are bad enough but at least you can open them directly. There are a number of tools around to convert OST to PST but I think they all cost. FWIW I have used Kernel tools a lot and they work well, their OST recovery tool
would surely do what you need.
That said if you just want to find something, they have a free OST viewer program, one of their free tools