Archiving old data
After a while of using OmniFocus every day, your database may become quite large, and especially if you synchronize with a mobile device, things could get quite slow. Most of the items are probably completed or dropped items that you rarely or never need to review, so OmniFocus includes a command to archive those old items to a different file, keeping your main database sprightly and nimble.
To archive your old data, choose Move Old Data to Archive from the File menu. A sheet appears with a date field; any items completed before that date, or dropped items that haven’t been changed since that date, will be moved to the archive.
OmniFocus saves your archive file in the same place as your main database: ~/Library/Application Support/OmniFocus/, where ~ is your home folder. If you ever want to look at your archived data, just choose Open Archive from the File menu; your archive opens in its own window.