DIPHTHONGS BEGINNING WITH O.
Oa, an improper diphthong, has the sound of open or long o; as in boat, coal, roach, coast, coastwise: except in broad and groat, which have the sound of broad a.
Oe, an improper diphthong, when final, has the sound of open or long o: as in doe, foe, throe: except in canoe, shoe, pronounced canoo, shoo. OE, a Latin diphthong, generally sounds like open e; as in Antoeci, foetus: sometimes, like close or curt e; as in foetid, foeticide. But the English word f~etid is often, and perhaps generally, written without the o.
Oi is generally a proper diphthong, uniting the sound of close o or broad a, and that of open e; as in boil, coil, soil, rejoice. But the vowels, when they appear together, sometimes belong to separate syllables; as in Stoic, Stoicism. Oi unaccented, sometimes has the sound of close or curt i; as in avoirdupois, connoisseur, tortoise.
Oo, an improper diphthong, generally has the slender sound of o; as in coo, too, woo, fool, room. It has, in some words, a shorter or closer sound, (like that of u in bull,) as in foot, good, wood, stood, wool;—that of close u in blood and flood;—and that of open o in door and floor. Derivatives from any of these, sound as their primitives.
Ou is generally a proper diphthong, uniting the sound of close or curt o, and that of u as heard in bull,—or u sounded as oo; as in bound, found, sound, ounce, thou. Ou is also, in certain instances, an improper diphthong; and, as such, it has six different sounds:—(l.) That of close or curt u; as in rough, tough, young, flourish. (2.) That of broad a; as in ought, bought, thought. (3.) That of open or long o; as in court, dough, four, though. (4.) That of close or curt o; as in cough, trough, lough, shough: which are, I believe, the only examples. (5.) That of slender o, or oo; as in soup, you, through. (6.) That of u in bull, or of oo shortened; only in would, could, should.
Ow generally sounds like the proper diphthong ou,—or like a union of short o with oo; as in brown, dowry, now, shower: but it is often an improper diphthong, having only the sound of open or long o; as in know, show, stow.
Oy is a proper diphthong, equivalent in sound to oi; as in joy, toy, oyster.