OST or Offline Storage Files represent user's mailbox data in offline mode on local machine upon registration with Exchange Server using Microsoft Outlook. It is automatically created on the first use of Microsoft Outlook upon connectivity with server. Once the file is created, the data is synchronized with the email server so that it is available offline as well in case of disconnectivity from email server.
PSD, Photoshop Document, represents Adobe Photoshop's native file format used for graphics designing and development. PSD files may include image layers, adjustment layers, layer masks, annotations, file information, keywords and other Photoshop-specific elements. Photoshop files have default extension as .PSD and has a maximum height and width of 30,000 pixels, and a length limit of two gigabytes.
You can also convert OST into many other file formats. Please see the complete list below.