If you want to convert OST to PST manually, you should follow the steps that are given below -
Launch MS Outlook.
· Go to the File tab, click on the Open & Export tab>Import/Export option.
· Now, select Export as a file option and click on the Next button.
· In the Export to a File wizard, select Outlook Data File (.pst) and hit the Next button.
· Now, choose the desired folder you wish to convert and click on the Next button.
· Click on the Browse button to save the resultant PST file and click on the Finish button.
In manual methods, users can lose their data, so I recommend you use a professional tool like DRS OST to PST Converter. It is a risk-free tool that converts OST files into multiple file formats. Also, its a user-friendly software and has a simple GUI.