O

Open Firmware: Cross-platform standard for controlling hardware. Open Firmware is used by all PCI-based computers running the Mac OS.

open source: Software that is developed, tested, and improved through public collaboration and distributed with the intent that it be shared with others.

ORBS (Open Relay Behavior-Modification System): A database, accessible via DNS lookups, that tracks known spammers (senders of junk mail). It contains SMTP servers that are known to allow third-party relay; senders of junk mail use these servers to forward their junk mail.