Russian orthography most often does not reflect vowel reduction, which can confuse foreign-language learners, but some spelling reforms have changed some words.
Nevertheless, in rapid colloquial speech they both may be reduced to schwa [ə],[1] for example, до́брым [ˈdobrɨ̆m] ('kind', instrumental case, singular masculine neuter) versus до́бром [ˈdobrəm] ('kind', prepositional case, singular masculine neuter).
The phenomenon is called akanye (аканье), and some scholars postulate an early tendency towards it in the earliest known textual evidence of confusion between written "a" and "o" in a manuscript that was copied in Moscow in 1339.
The yekanye pronunciation is coupled with a stronger tendency for both unstressed /a/ and /o/, which are pronounced the same as /e/ after /j/ and palatalised consonants.
Yakanye (яканье) is the pronunciation of unstressed /e/ and /a/ after palatalised consonants preceding a stressed syllable as /a/, rather than /i/ (несли́ is pronounced [nʲasˈlʲi], not [nʲɪsˈlʲi]).
This pronunciation is observed in Belarusian and in most Southern Russian dialects, as is expressed in a quip (with liberal yakanye): That example also demonstrates other features of Southern dialects: palatalised final /tʲ/ in the third-person forms of verbs, [ɣ] for [ɡ] and [w] for [u] (in some places) and [v], clear unstressed [a] for [ɐ] or [ə].
That is, in cases of doubt, codifiers of 1956 based their choice not on etymological conformity but on the spread of usage.
In the closely related Belarusian, the original /o/ has merged with /a/, like in Standard Russian, but the reduced pronunciation is reflected in the spelling.