O, the fourteenth letter, is in the oldest Runes, on the stone in Tune, and on the Golden horn figured by ᛟ, which was evidently taken from the Greek Ω the later common Runic alphabet in earlier monuments has no ó, but uses u or au instead, e. g. on the Jellinge stone in Denmark. Afterwards the Rune ᚮ, ᚭ, or ᚯ, ᚬ, appears under the name of óss in the Runic poems—óss er flestra ferða (= fjarða) = all firths have an óss (mouth). The form was evidently taken from the A. S. Runic ᚩ, which stands for a, and in A. S. is called ós, which answers, not to Norse óss (ostium), but to áss (= ans, i. e. the heathen gods); but the Norsemen or Danes in borrowing the Rune seem to have misinterpreted its name or mistranslated it from ignorance of the phonetic laws existing between the A. S. and the Norse. The ᚭ in Scandinavian Runic inscriptions is therefore a mark of later date (11th or 12th century).
B. PRONUNCIATION.—The o is either short (o) or long (ó); the former (o) is sounded like Engl. o in cod, the latter (ó) as in Engl. no, note; but the rules given at the beginning of the introduction to letter A (p. l) apply equally to this letter, bōð being sounded bawth, but krŏss in North. E. cross.
C. CHANGES.—In most of the oldest vellums o instead of u is used throughout in inflexions, -o, -or, -om, -on, -oð, -ot, -osk, -oll, -onn, instead of -u, -ur, -um … -unn (Gramm. p. xxxv, col. 1, A); afterwards both forms are used indiscriminately, till in the 15th century the u prevailed, and has kept its place ever since; whether there was a difference in sound, and what, we are unable to state. 2. so also in a few root words, goð, goll, fogl, oxi, skolu, monu, hogr, togr, monr (Dan. mon), smogoll, = guð, gull, fugl, uxi, skulu, munu, hugr, tugr, munr, smugull; on the other hand, is sonr (a son), but sunr the older form. 3. a and o or u interchange in the inflexions, fagnaðr, fognoðr, fögnuðr; kallan, kollon, köllun. 4. ú has changed into ó in the prefixed negative, ó-vitr for ú-vitr (unwise). 5. jú into jó, njóta, originally njúta; ljós, Swed. ljús: forms like mjókr for mjúkr, dókr for dúkr may also be found in vellums, but are very rare. II. ́, the vowel-change of á (see p. 1), is frequently spelt o (tor, nott, = t́r, ńtt), but was in sound different from ó proper, and has since disappeared from the language, although remains of this ‘umlaut’ still exist in nótt, ól, spónn (= ńtt, ́l, sṕnn), but this o is sounded exactly like common o. So also o and ö are confounded in MSS., bornom = börnom = börnum. For the absorption of consonants see Gramm. p. xxx, col. 1, and the words themselves.
☞ Owing to the inability of the Scandinavian languages to sound v (w) before a vowel of the u class, several root words, which in dictionaries of the cognate languages (Germ., Engl.) begin with w, are in the Icelandic to be found under o, as okr, orð, orka, ormr, Óðinn, óðr, ósk, ómr; as also j, in ok (jugum), ostr, and ok the conjunction.© Tim Stridmann