PST (Personal Storage Table) and OST (Offline Storage Table) files are associated with Microsoft Outlook:
PST files store email messages, contacts, calendars, and other mailbox data within Microsoft Outlook. They are used for POP and IMAP email accounts and local folder storage and archiving. Typically, PST files are stored locally on the user's computer or at a network location.
OST files are offline copies of a user's mailbox data synchronized with an Exchange Server. These files allow users to access their mailbox data even when not connected to the server. OST files are automatically created when Outlook is used in Cached Exchange Mode.