I have recently gone through an informative article that provides the two different approaches to import Outlook OST to Office 365 mailbox. You can also refer to this article in case you are searching for an advanced and reliable solution. However, in case you are willing to accomplish this task using the manual methods then you need to know that you have to convert the OST files into PST and then you can import those PST files into Office 365. This process is precisely explained in this article.
The main drawback of this manual approach is that it is quite complex and takes a lot of time. Apart from this, there is a high chance of data loss in case a user is a non-technical person. Therefore, experts have always advised opting for a safe and secure OST to PST Converter to perform the conversion and migration of your OST files.