Hello,
You can convert your OST files to PST by forwarding your Emails. but this option is only good when you don't have so many OST files. Just email your PST items to your personal email address. Just select the items, then right click and choose Forward. If you have many OST files, you could drag the files to a new folder in My Documents, then just zip the folder and email that zip file.
If this manual methods works for you then well and good or else definitely the alternative way is to use OST to PST converter to convert Outlook OST to PST in no time and in a very secure way.