If you have an OST is that you have Exchange, likely Exchange Online through Hosted Exchange or Office 365. Once you Import a PST from within Outlook (File | Importů and then Import items into the current folder), then that inherently merges the two files, although you might have to move some folders around afterward. You have to this for each user. If there are many users then you might be best to use a third-party migration service like vMail OST to PST migration tool.
Know more info- https://www.vsoftware.org/blog/fix-e...-ost-file.html