I just want to give you some introduction about this. Actually there are two kinds of お-words, honorific/humble expression and refined expression. honorific/humble expressions are to show respects to listeners. refined expressions are to add some elegance or politeness to words, not to show respects to others. ■honorific/humble-お（ご） ・お元気ですか？ おげんきですか？ How are you? ・お電話してもいいですか？ おでんわしてもいいですか？ May I call you? *I guess, as far as you use polite endings, sentences sound polite enough. げんきですか？／でんわしてもいいですか？ So, unless you are an advanced Japanese speaker, you don't have to use these お yet. ■elegant-お（ご） ・毎日、お茶をたくさん飲みます。 まいにち、おちゃを　たくさん　のみます。 I drink Japanese tea every day. ・ご飯をたくさん食べたい。 ごはんを　たくさん　たべたい。 I want to eat a meal a lot. ・わたしは、お菓子が大好きです。 わたしは、おかしが　だいすきです。 I love sweets. *Most people use お茶、ご飯. (If someone omits お like 茶 or 飯(めし), it would sound rough.) It depends, but most female persons use　お菓子、お店（みせ）、お箸（はし）、お寿司（すし）. Basically, お would be added in front of 和語（わご）, whereas ご would be added in front of 漢語（かんご), but there are exceptions. So, over all, I recommend you to start with elegant-お. I hope this helps you a bit!

Hi Maxine, Indeed, O shows politeness. But as René mentioned, this matter is very difficult to be covered comprehensively, because there are a lot of words that should be added "O" and should not do. And this mostly depends on the common sense of Japanese. So, even if you, as a foreigner, do not add "O", Japanese do not care about it at all. Finally let me answer your question as much as possible. Situation: (1) when you talk with people who you respect or you should respect (even if you do not). (2) in case that adding O is an usual way. Words: (1) 酒、元気、上手、薬、、、 (2) 店、祭り、菓子、酒、、、 To avoid misleading, I show you the words in Japanese. Please check on your dictionary. Good luck!

O-genki is used for your respect person in front of you. Sake is valuable drink for Japanese.So you have to put O before Sake. As you notice,Genki and Sake are Noun． Than you.

We Japanese add 'O' when we have to talk politely or in formal situations we talk with superiors, etc.

okay, thanks:)