There is any number of adjectives that end in "ой": большой, молодой, etc. It is nominative masculine ending.
Please notice the vocative case (Оче), and the structure of the prayer being very close to the original Church Slavonic. A modern Russian version, while outwardly structurally more similar to CS, departs grammatically and otherwise from the CS version.
However, I find the modern evrsion somewhat awkward. FWI, the Serbian letter "ђ" is equivalent to Russian "дъе" in other words a palatalized "d" (as in dye).
For example "нека се свети" can be stated affirmatively also as "да се свети Име Tвоје" or even "да свети се име Твоје".
Instead of "нека дође Царство Твоје" can be stated just as well "да приђе Царство Твоје", and instead of "нека буде воља твоја" can be stated "да буде воља Твоја."
The order of Heaven and earth is reversed because of the western influence.
The translation into modern Serbian is also somewhat inconsistent. For example it says "Оче наш" and "Хлеб наш" but "дужницима својим" instead of "нашим" (svoy and nash both mean our). Why would they pick one and n to the other is an enigma.
"и не уведи нас у искушење" in Serbian all initial "в" prepositions have been morphed into "y" (easier to pronounce, such as Уторак for Вторник). In the Serbian recession of the CS, the word "введи" was "ваведи." here we have this half sound issue again and how to spell them.
In the 16th century they didn't really distinguish the "ь" from "ъ" and didn't use it as in Bulgarian, but I would imagine "въведи" was the option in some recensions later on.
For instance, the 16th c. Serbian version also say "дългы" for debts and "дължникомъ" for debtors.
It also uses the word "напастъ" instead of "исушение".
There is any number of adjectives that end in "ой": большой, молодой, etc. It is nominative masculine ending.
In Serbian, on the other hand, the endings are the same as in CS, in nominative бољи (better) млади (young).