OST File

An OST file (.ost, Offline Storage Table) is the local cached copy of an Exchange mailbox used by classic Outlook for Windows when running in cached mode. The OST lives in %LOCALAPPDATA%\Microsoft\Outlook\ and grows with the mailbox content. Cached mode improves performance and enables offline access, but OST files can balloon to multiple gigabytes for large mailboxes, slow VDI / shared workstations, and occasionally corrupt. The new Outlook for Windows doesn't use the same OST model — it's a thin client that talks to Exchange via Graph, with much smaller local cache. OST files are different from PST files, which are user-managed archive containers.