1.When letter "o" is stressed, you need to pronounce "o": что́б (in order to), мо́ст (bridge), до́м (house), ло́дка (boat). 2.When letter "o" is unstressed you need to pronounce "a" + "o" together (about 75 - 80% from "a" + 20 - 25 % from "o" ) : хорошо́ (good), прости́ть (to forgive), слова́рь (dictionary), де́рево (tree), спаси́бо (thanks). You can check your pronounciation on: 1) http://lingvopro.abbyyonline.com/ru/Search/ru-en and 2) http://www.translate.ru/ The words are pronounced by native speakers there and pronunciation of words is more correct than on http://translate.google.com where the words are pronounced by robot.

Leonid is absolutely right: stressed "o" is just "o" and unstressed "o" is close to "a". You can always check a pronunciation at www.forvo.com - a pronunciation dictionary.

Leonid is right! But at some places of Russia, people say "horosho". I know one please "Нижний Новгород". So if you speak a lot of "o" it is normal, people can understand you.

о sounds like a in the syllables which preceed the stressed syllable and in the beginning of the words (the syllable in this case is also unstressed). e.g. молоко - [мэлакО] огурец - [агурЕц] The other unstressed letters о are pronounced as [а] with a slight glide which sounds like [э]. e.g. хорошо - [ха(э)рашО] город - [гОрэт]

Also you should bear in mind that not all native speakers pronounce it that way. In fact, there's a big number of speakers who pronounce "o" just as it's written. You better stick to what you've been told by Leonid and Eugenia, but be ready to hear from some people "khorosho" instead of "kharasho", "prosti" instead of "prasti" and so on....