To create ost file in Office 365, user can go for
SysInspire ost to Office 365 Converter Software:
Step 1: Install SysInspire ost to office 365 Converter.
Step 2: Click on the "Browse" button and select the ost file which you wanted to create in the Office 365 layout.
Step 3: Select the scanning mode to handle the error if any in the EDB file.
Step 4: Click on the Next button to preview the contents within the ost file whether it is the same ost file that is to be created for Office 365.
Step 5: Select Save as Office 365 options and Click on Next button.
Step 6: Click on Browse button, and choose a location to save the converted ost file.
Now, I am sure you will get the ost file created in Office 365 format.
More info:- Convert OST to PST