The two first /o/s are borrowed as /a/ in Japanese. I don't understand why the Russian /o/ vowel sounds like /a/ to Japanese speakers instead of /o/ which is a phoneme in Japanese.
Because хорошо is pronounced just like ハラショ.
First of all, loanwords are only based on a word in a different language; they do not have to be duplicates. Russian and Japanese are different languages, especially in terms of pronunciation, so not everything will go between them smoothly.
I am not an expert in the Russian language, but if I remember correctly, the letter 'o' sounds like an unstressed 'a' (or a schwa) when it is unstressed, and when it is stressed, it sounds like an 'o' as usual. According to dictionaries, the word 'хорошо' has stress on the last 'o', so the last one will sound like an 'o' but the first two, which are unstressed, will sound sort of like an 'a'. That could explain why the first two are borrowed as /a/ but the last is an /o/.
So it sounds like /a/ to Japanese speakers because that's pretty much what it is.
This probably belongs to the Russian Language SE, but it is due to palatalization.
This site has a brief intro to pronunciation: http://www.russianlessons.net/lessons/lesson1_main.php
When stressed, it is an "oh" sound and when unstressed is it an "ah" sound.
Japanese language tends to transcribe words as they sound, thus ハラショー
@macraf: хорошо in IPA is xərɐˈʂo. – Oct 24 '15 at 0:02
@blavius: it is due to palatalization.
Not so simple. The first consideration is stress. Unstressed /o/ and /a/ merge to /a/. (Akanye vs. okanye.)
The choice of allophone then brings in other factors.
• [ɐ] (or [ʌ]) for the syllable preceding the stressed one. (Further reduction less likely.)
• [ə] (the most frequent vowel in English) for other unstressed syllables.
For more phonetic detail than you may want or ever need to know