The Oblique Strategies are a deck of cards. There are four versions. The first three editions (1975, ’78, ’79) were regularly on sale, the fourth (1996) is completely different, multilingual and printed for private use only.
They measured about 2-3/4″ x 3-3/4″. They came in a small black box which said “OBLIQUE STRATEGIES” on one of the top’s long sides and “BRIAN ENO/PETER SCHMIDT” on the other side. The cards were solid black on one side, and had the aphorisms printed in a 10-point sans serif face on the other.
The deck itself had its origins in the discovery by Brian Eno that both he and his friend Peter Schmidt (a British painter whose works grace the cover of “Evening Star” and whose watercolours decorated the back LP cover of Eno’s “Before and After Science” and also appeared as full-size prints in a small number of the original releases) tended to keep a set of basic working principles which guided them through the kinds of moments of pressure – either working through a heavy painting session or watching the clock tick while you’re running up a big buck studio bill. Both Schmidt and Eno realized that the pressures of time tended to steer them away from the ways of thinking they found most productive when the pressure was off. The Strategies were, then, a way to remind themselves of those habits of thinking – to jog the mind.
Eno ad Schmidt said:

These cards evolved from our separate observations on the principles underlying what we were doing. Sometimes they were recognized in retrospect (intellect catching up with intuition), sometimes they were identified as they were happening, sometimes they were formulated.
They can be used as a pack (a set of possibilities being continuously reviewed in the mind) or by drawing a single card from the shuffled pack when a dilemma occurs in a working situation. In this case,the card is trusted even if its appropriateness is quite unclear. They are not final, as new ideas will present themselves, and others will become self-evident.

This is the Oblique Strategies site. You can also consult the deck online.