A command interperter used by all Sun Microsystems boxen. Implemented in firmware, its purpose is to allow finegrained control of the boot procedure and provide some pre-boot hardware diagnostics. Strictly, OBP is a forth interpreter, but with several canned scripts to do most anything one might need to do. OBP is characterized by it's "ok " prompt.

A couple of commands I remember:

boot [device] [boot args]
Boot the machine. device is usually an alias created with nvalias. Boot args are passed to init, things like -s for single user, -r for reprobe/reconfig, etc.
nvalias [device path] [alias]
Create device aliases. "disk" is easier to type than "/pci/pci@00ff/dc-001c@0,1" etc. usually used with ^Y and the show-disks/show-nets commands.
Prints a list of all detected disks (actually disk controllers) connected, labeled a, b, c, etc, and prompts you to enter one of the labeled letters. Entering that letter will put the associated device name in the ^Y cut/paste buffer, allowing easy use with nvalias.
Same idea as show-disks, but with network interfaces.
Probe all detected scsi cards for connected disks & print out their manufacture's information. Useful for troubleshooting bad disks, cables, etc.
test [device]
Perform a self test on the sepecified device. Handy on network interfaces, as it will test both the card and show the link status.
Watch for packets. Will print a . for each good packet recieved, & an x for each bad packet. test net tells you if you have link, watch-nets tells you if you're actually seeing packets.
Reset all OBP options and paremeters to their factory defaults. Useful for restoring a confused, fubar'd OBP to normal working order. A last resort on a mysterious misconfiguration, or a handy quickfix when maintaining an army of Sun boxen.