If you want to export ost to office 365 then there are two methods available.
1.Indirect method
2.direct method
1. Indirect method is way to process where the ost file moving into pst then
Importing pst files into office 365. Indirect method is basically a free method
which is very time consuming and you should be technical person to deal with
it.
2. Direct method is the process where you can easily import OST files into
Office 365 by using some third party OST to Office 365 Migration tools. This is
quite simple way to export OST to Office 365. Use this OST to Office 365
Converter software which has very simple and enhanced user friendly interface to
help the users to easily convert all offline ost files into office 365 without
any hassle. You can also use it free demo version to check it features and
function to understand this tool.
Read more info:-
http://www.osttopstconverter-tool.com/ost-to-office365-migration.html