Most nouns ending in o preceded by a vowel are pluralized by simply adding an "s" on the end of the word; for example:
You form the possessive of plurals ending in "s" by adding an apostrophe: the computers' network connections, the rabbits' ears.
Some irregular plurals are geese, men, children, cacti, data, or media.
The possessive of all English plurals ending in -s is formed by adding an apostrophe after the final s: the Joneses' house. (Do not mistake singulars that end in -s, like Jones or Charles for plurals).
For nouns ending in 'y' preceded by a consonant, the plural is formed by dropping the 'y' and adding 'ies' to the end of the word.For nouns ending in 'y' preceded by a vowel, the plural is formed by adding an 's' after the ending 'y'.Examples:baby, babiesparty, partiesstory storiesboy, boysplay, playsstray, strays
The noun codex has the plural codices.(Plural is formed in the same way as index, which has plurals indices and indexes.)
A regular plural is a noun to which an -s or an -es is added to form the plural. An irregular plural is plurals formed in some other way. There are many forms of irregular plural nouns.Nouns ending in the letter -o, sometimes they are pluralized by adding -s, while other words must be pluralized by adding es. These words must be memorized, because there is no simple rule to explain the differences. (echo, echoes or hero, heroes)Most nouns ending in o preceded by a vowel are pluralized by simply adding -s. (photo, photos or tattoo, tattoos)Some nouns ending in the letter -y, replace the ending y with -ies. (baby, babies or story, stories)Nouns ending in -ypreceeded by a vowel (a complex vowel sound), an -s is simply added, as usual. (day, days or turkey, turkeys)Some nouns ending in -f or -fe, replace the ending -f or -fe with -ves. (life, lives or knife, knives)Some nouns change the vowel sound in becoming plural. (man, men or goose, geese)Some plurals are left over from Old English; many plurals come from other languages. Examples of these irregular plural nouns are:singular / pluralchild / childrenox / oxencactus / cactialumnus / alumnidatum / datamedium / mediaoasis / oasesbasis / baseslarva / larvaeantenna /antennaeindex / indecesperson / people
The noun with no vowel is 'rhythm'.
If you mean a regular plural noun as opposed to an irregular plural noun, then the answer is that a common noun can be a regular plural or an irregular plural; and a proper noun can be either a regular plural or an irregular plural. The difference between how a regular and an irregular plural noun is formed is usually based on the origin language from which it came to the English language. A common noun is a word for any person, place, thing, or idea. A proper is the name of a specific person, place, thing, or a title. The regular plurals are formed by adding an 's' or an 'es' to the end of the singular noun to make it a plural noun. The irregular plurals are quite varied. See the link below for a simple step by step list for using irregular plurals.
It says in the rule of forming singular noun to plural add "S" to nouns ending in "O" preceeded by a vowel .
The plural is a normal s plural, valleys.Words ending in a vowel and Y usually form normal plurals by adding S. It is only the words ending in consonant and Y that change the Y to IE and add S. The same applies to verb forms.Examples:monkey - monkeyscandy - candiesobey - obeysmarry - marries
The plurals form for the noun subsidiary is subsidiaries.
There are two accepted plurals for the noun madam: madams or mesdames.