Orthodox Wiki Page.https://orthodoxwiki.org/Lord%27s_Prayer#Old_Church_Slavonic
Interesting issue, because your question itself is not correct and correcting it shows the best answer. The correction is that Orthodox Churches do not sing "Russian", they sing an updated version of "Church Slavonic".
The Old Church Slavonic here is:
Оч͠е нашь ижє ѥси на н͠бсєхъ . да с͠титьсѧ имѧ твоѥ
да придєть ц͠рствиѥ твоѥ
· да бѫдєть воля
твоя · яка на н͠бси и на земли хлѣбъ нашь насѫщьиыи ·
даждь намъ дьньсь · и остави намъ · длъгы
нашѧ · яко и мы оставляємъ длъжникомъ нашимъ
и нє въвєди насъ въ напасть · иъ избави ны отъ
In Church Slavonic, the letter is ѥ
That is a conjunction of English y and e, or in modern Russian йэ
Old Church Slavonic uses ѥ , prounced йэ.
Updated Church Slavonic uses е, pronounced йэ
Modern Russian uses ё, writes it as ё or е, pronounces it as йо.
We go by the Updated Church slavonic, and you can check it in this choir song in Moscow:https://www.youtube.com/watch?v=wbeZqnNCUOk
The way they are singing it would be wrong in their modern Russian, so they are singing it not in modern Russian, but in Church Slavonic.