This word strikes me as a bit of an ugly doo-dad, a knot of voiced stops. I’m not a synesthete, but I see a kind of sick yellow colour with it (your results may vary). I get that from a coagulation of different factors: the gd, which always seem to be glued together or tied with gut strings; the two o’s, which can look kooky or pretty or clean and cold but which always stand for back vowels, more often than not rounded, and so have a sort of dullness to them; the oa, which in some cases can be quite lovely but which seem stunted by the d at the end.
Aside from that, the word makes me think first of the name Ogden, which has a few associations for me: an industrial district of Calgary; Ogden Nash, an amusing poet; David Ogden Stiers, who played Charles Emerson Winchester III on M*A*S*H. And it makes me think of dog (and dead dog), dad, good ad, d00d…
Of course, all of that might just make you say, “Huh.” On the other hand, the word ogdoad itself might make you say Huh… and Hauhet, Naunet, Nu, Amaunet, Amun, Kauket, and Kuk: the ogdoad of ancient Egypt. They were male-female pairs (the males had the shorter names) of gods representing the concepts of primordial waters; air or invisibility; darkness; and eternity or infinite space. Of course there were four; that’s the number of completeness, and they were gods of creation. Double that and you get the most excellent number eight (also thought lucky in Chinese, but for paronomastic reasons).
And the Gnostics liked this double quaternity, too, speaking of the four emanations by which creation occurred: masculine abyss and feminine silence, grace, thought; from these, masculine mind and feminine truth; from these, masculine word and feminine life; and from these, masculine man and feminine church.
At any rate, it’s eight. You likely know the root for “eight” as octo. Well, for the ordinal, there was a phonological transformation in Ancient Greek, making those stops voiced, and from that came the word for a set of eight – just as myriad is a set of ten thousand. Interestingly, this is not some recent borrowing into English that really ought to be set in italics or quotes; it’s been around since medieval times – in fact, there’s even a sighting of it in Old English.
The ancient Angles were talking about Gnostics and hieroglyphics? Some were, yes, but others made broader use: “The Ogdoad, they said, was the first Cube, and the onely number evenly even under ten” (T. Stanley, 1660). An ogdoad, after all, can be any eight things, as long as you don’t mind that your readers will likely take it as a reference to the Egyptians or the Gnostics. Or, more likely, they will just say, “Huh?”