お～ & ご～ Polite Language
丁寧語 (ていねいご・Polite Language)・美化語 (びかご・Refined Language)
Two of the major components of polite and refined language (丁寧語 and 美化語) are the prefixes お～ and ご～. Both can be represented by the kanji 御 (and in some cases it can also be pronounced おん). There is no hard and fast rule as to which words do and don't use お～ or ご～ so in general you need to memorize which words can have this prefix as well as which reading to use (although often it is written in kana). However, there are a few general principles which can help you to choose.
お～
Language used to make one's language more refined. Used at the beginning of nouns and adjectives. It can also be used with verb stems and adverbs in certain situations. In general, お～ attaches to nouns that are native Japanese words (和語, わご), but some words of Chinese origin (漢語, かんご) can also take this prefix. Words with this prefix are often related to words used in daily life, such as food and drink, or things related to everyday tasks.
ご～
A word placed at the beginning of a noun or adjective to indicate politeness. Only used for kanji compound words in which the first character is an on'yomi character, therefore words of Chinese origin (漢語, かんご). The words attached to this prefix are often abstract nouns, and not physical objects. There are a few native Japanese (和語, わご) adverbs and adjectives that can use this prefix. While お～ is usually in kana, ご～ is most commonly written in kanji.
Integrated Words
お and ご have become such an integral part of some words, that they cannot be removed and do not add politeness or respect to the word.
Examples:
おにぎり onigiri, rice ball
おはよう good morning
おなら fart
ごめん sorry
ごはん meal, rice
Deciding whether to use お～ or ご～ or no prefix at all
お～ and ご～ are often used to convey a feeling or respect or politeness towards the person you are addressing, so while you often use it with words that belong to the person, words that don't belong to the person are less commonly heard. お車 (おくるま) can be used to talk about a superior's car, but you wouldn't say お飛行機 (おひこうき) or お電車 (おでんしゃ) because those don't belong to them.
お～ and ご～ aren't used with katakana words, so you won't find it used with loan words. However, おトイレ can be used, probably because it is an everyday word.
What about おん?
In spoken formal Japanese, 御 is pronounced as おん when the word it is attached to is referring to an organization. For example: 御社 (おんしゃ, your (someone else's) company), and 御校 (おんこう, your (someone else's) school). [Note: In formal writing, 貴社 (きしゃ, your company) and 貴校 (きこう, your school) are used.] You can also read 御礼 (gratitude) as either おれい or おんれい, with おんれい being the more formal of the two.
Oh wait, I forgot to mention み～
A much less common reading of the prefix 御 is み, which is specifically used when talking about or to someone who deserves the highest respect - Buddha, God, the emperor, members of the imperial family. 御心 is usually read as おこころ but is read as みこころ when referring to the 心 (heart/love) of Buddha, God, or the emperor. 御国 (みくに) is country, 御名 (みな) is name, 御手 (みて) is hand, 御顔 (みかお) is face, and 御霊 (みたま) is spirit when referring to these entities of high respect.
And then there is ぎょ～
When 御 is read as ぎょ it shows respect to the emperor and his family, whether it's used as a prefix or not. 崩御 (ほうぎょ) is used only to refer to the death of the emperor or a member of the imperial family. Most of the words using ぎょ are very specialized, however a few are known in mainstream use. 御意 (ぎょい) originally was used to talk about the will of the emperor, but came to mean "as you wish" and can be used for anyone of a higher status than you. It is in use today, but is very formal and a bit old-fashioned. It's most common in writing. 御者 (ぎょしゃ) originally meant a coachman who drove a horse-drawn coach or carriage for the emperor and the imperial family, but today it is a general word for coachman.
Examples
Now to the examples!
Nouns
お名前 おなまえ name
ご家族 ごかぞく family
Adjectives
お優しい おやさしい kind
ご丁寧 ごていねい polite
Adverbs
お近く おちかく near
ごゆっくり take your time
Verbs
Used with the pattern お・ご～する or お～になる
お預かりする おあずかりする to look after
ご活躍する ごかつやくする to be active
お食べになる おたべになる to eat
My textbook: 外国人のための日本語敬語の使い方基本表現85
