I've tried versions like "retaining the unstressed “o” and “a”" and it seems to be wrong.

  • Hi and welcome to Russian SE. Unfortunately - and if you take a look at your own question it will be quite obvious - this is not a question about Russian language and this renders it - and actually all "how this Russian term can be translated to English" questions - off-topic. – shabunc Feb 4 '20 at 9:59
  • Окающие и акающие: speakers of the dialects without reduction of unstressed vowels o and a; those who speak with okanye (literally o-ing) and with akanye (literally a-ing). Check en.wikipedia.org/wiki/Akanye Actually it depends on the context where to use the translation: maybe, "those who speak rustic and old fashioned Slavic dialect" provides a sufficient sense and meaning of this phrase. – Jan Marozaŭ Feb 5 '20 at 16:18