How to open an OST file?

Im having a problem with my outlook. How can I open my OST file? They’ve already deleted my email and I beleive my OST is already orphaned coz theres is no way i can synchronize them right?. I cannot open them anymore with my outlook coz ive change to a new server. Are there still a way for me to get my mails from my old OST file? Or can i convert my old OST file to a PST or transfer it to my new mail using the old OST?
I hope someone could help me… THANKS!