The moderator's suggestion, the JewishGen's Given Names Databases
https://www.jewishgen.org/databases/GivenNames/ is the most easily
accessible reference for given names used by our ancestors and is
pretty comprehensive. There are also entire books on the topic, such
as the one by Alexander Beider on Ashkenazic given names.

Ovshiy, Yishea, and Joshua all refer to the same Hebrew name,
Yehoshua. However, Ovshiy was not your great-grandfather's "Russian"
name but a Yiddish nickname used in the Russian Empire. Speaking very
generally and briefly, in 19th century Russia most of our ancestors
had a Hebrew name used for religious and official purposes, a Yiddish
name used for everyday purposes, and a collection of nicknames based
on the Yiddish name. Truly Russian names were generally not used by
our ancestors until the 20th century and most of them would appear
familiar even if the spelling is a little different >from the English
version, e.g. Catherine, Nicholas, Alexander, etc.

It is a good idea to read an explanation of given name traditions in
JewishGen, such as the one at
https://www.jewishgen.org/databases/GivenNames/nature.htm, so you will
understand our ancestors' use of Hebrew names, Yiddish names, and
nicknames.

Because there were so many different nicknames for each name and so
many variations in spelling for each Hebrew and Yiddish name,
especially after they were translated >from a different alphabet
(Russian Cyrillic or Hebrew), it was not possible for the JewishGen
Given Name Database to include them all the possible spellings, so do
not get discouraged if your first search is unsuccessful. Instead, try
some alternative spellings. For example, Ovshiy is not included in the
Given Name Database for Lithuania but Hoshiye, Hovsey, Hovshe, Oshaye,
Osheye, Osiye, Osyas, Ovshe, Yeshiye and over a dozen others are
included.

The JewishGen Given Name Database is organized by modern-day country,
so it will help if you know where in the Russian Empire your maternal
great-grandfather was from.

Good luck - Judith Singer