Anderson's Three Hearts And Three Lions (London, 1961) informs us of at least two other worlds where Law and Chaos fight:
"'Those two worlds - and many more, for all I know - are in some way the same. The same fight was being waged, here the Nazis and there the Middle World; but in both places, Chaos against Law, something old and wild and blind at war with man and the works of man.'" (p. 155)
Thus, it makes sense to read Three Hearts... before the Operation... books.
Law and Chaos also fight in another fictional multiverse and, in a third, they are called Order and Chaos.
The fight of the Old Life against the works of man is also in Hrolf Kraki's Saga (see here) and in The Merman's Children (see here). The Old Ways are threatened by the ways of men in A Midsummer Tempest (London, 1975), pp. 49-51.
In the first interlude in Operation Chaos (New York, 1995), Steve Matuchek, still addressing hypothetical other universes, says:
"...the big events do matter to you. He's also your Adversary." (p. 48)
Does every universe have an Adversary and, if so, is he the same in each?
The Adversary, soliloquizing, anticipates that Matuchek and Graylock may be a grave danger to his great plan. He states as much later when he directly addresses Matuchek. Do we see this "'...Plan...'" (p. 76) either implemented by the Adversary or endangered by the Matucheks?