OST to Office 365 Converter Tool is a software application or online service used to convert email messages and other data stored in an Offline Storage Table (OST) file to Microsoft Office 365. Microsoft Outlook uses OST files to store email messages, calendar events, tasks, and other data when the email account is configured in the "Cached Exchange Mode."

The purpose of an OST to Office 365 Converter Tool is to allow users to migrate their email messages, contacts, and other data from an OST file to Office 365, where they can access their data from anywhere and on any device. This can be useful for businesses and individuals who want to move their email data to the cloud for greater accessibility and security, or who need to transfer their data to a different email service.

An OST to Office 365 Converter Tool typically works by reading the data stored in an OST file and converting it into a format that can be imported into Office 365. The tool typically provides options to convert the data into different Office 365 services, such as Exchange Online, OneDrive for Business, or SharePoint.  OST to Office 365 Converter Tools also provide additional features, such as preserving metadata and other important information or converting the data into a different file format.