In which situations and next to which words do i have to put on an "o-" as in o-sake and o-genki?
In which situations and next to which words do i have to put on an "o-" as in o-sake and o-genki?

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Answer

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!

Answer

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!

Answer

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.

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

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okay, thanks:)