:obo spark A spark of the form O.O (so called after its rle encoding).

:octagon II (p5) The first known p5 oscillator, discovered in 1971 independently by Sol Goodman and Arthur Taber. The name is due to the latter.

	...OO...
	..O..O..
	.O....O.
	O......O
	O......O
	.O....O.
	..O..O..
	...OO...

:octagon IV (p4) Found by Robert Wainwright, January 1979.

	.......OO.......
	.......OO.......
	................
	......OOOO......
	.....O....O.....
	....O......O....
	...O........O...
	OO.O........O.OO
	OO.O........O.OO
	...O........O...
	....O......O....
	.....O....O.....
	......OOOO......
	................
	.......OO.......
	.......OO.......

:octomino Any 8-cell polyomino. There are 369 such objects. The word is particularly applied to the following octomino (or its two-generation successor), which is fairly common but lacks a proper name:

	..OO
	..OO
	OOO.
	.O..

:odd keys (p3) Found by Dean Hickerson, August 1989. See also short keys and bent keys.

	..........O.
	.O.......O.O
	O.OOO..OO.O.
	.O..O..O....
	....O..O....

:omino = polyomino

:omniperiodic A cellular automaton is said to be omniperiodic if it has oscillators of all periods. It is not known if Life is omniperiodic, although this seems likely. Dave Buckingham's work on Herschel conduits in 1996 (see My Experience with B-heptominos in Oscillators) reduced the number of unresolved cases to a finite number. At the time of writing the only periods for which no oscillator is known are 19, 23, 27, 31, 37, 38, 41, 43 and 53. If we insist that the oscillator must contain a cell oscillating at the full period, then 34 and 51 should be added to this list. The most recently achieved periods are 49, a glider loop which Noam Elkies constructed in August 1999 using p7 reflectors built from his new p7 pipsquirter, and 39 (formerly only possible without a p39 cell), which Elkies contructed in July 2000.

:onion rings For each integer n>1 onion rings of order n is a stable agar of density 1/2 obtained by tiling the plane with a certain 4n × 4n pattern. The tile for order 3 onion rings is shown below - the reader should then be able to deduce the form of tiles of other orders.

	......OOOOOO
	.OOOO.O....O
	.O..O.O.OO.O
	.O..O.O.OO.O
	.OOOO.O....O
	......OOOOOO
	OOOOOO......
	O....O.OOOO.
	O.OO.O.O..O.
	O.OO.O.O..O.
	O....O.OOOO.
	OOOOOO......

:on-off Any p2 oscillator in which all rotor cells die from overpopulation. The simplest example is a beacon. Compare flip-flop.

:O-pentomino Conway's name for the following pentomino, a traffic light predecessor, although not one of the more common ones.

	OOOOO

:Orion (c/4 diagonally, p4) Found by Hartmut Holzwart, April 1993.

	...OO.........
	...O.O........
	...O..........
	OO.O..........
	O....O........
	O.OO......OOO.
	.....OOO....OO
	......OOO.O.O.
	.............O
	......O.O.....
	.....OO.O.....
	......O.......
	....OO.O......
	.......O......
	.....OO.......

	.OO..........
	OO...........
	..O..........
	....O....OOO.
	....OOO....OO
	.....OOO.O.O.
	............O
	.....O.O.....
	....OO.O.....
	.....O.......
	...OO.O......
	......O......
	....OO.......

:orphan Conway's preferred term for a Garden of Eden.

:oscillator Any pattern that is a predecessor of itself. The term is usually restricted to non-stable finite patterns. An oscillator is divided into a rotor and a stator. See also omniperiodic.

In general cellular automaton theory the term "oscillator" usually covers spaceships as well, but this usage is not normal in Life.

:overcrowding = overpopulation

:over-exposure = underpopulation

:overpopulation Death of cell caused by it having more than three neighbours.

:overweight spaceship = OWSS

:OWSS A would-be spaceship similar to LWSS, MWSS and HWSS but longer. On its own an OWSS is unstable, but it can be escorted by true spaceships to form a flotilla.

:Ox A 1976 novel by Piers Anthony which involves Life.