No one knows what was originally written. No one alive, that is.
I do not know which source was used for the NWT OT.
The text that the KJV NT is based on is known as the Textus Receptus, prepared in some haste by Erasmus. I leave you to Google for information about him and that text.
A couple of 19th century Christendom priests decided they did not like the Textus Receptus, so they produced their own. Their surnames were Westcott and Hort. It is very interesting to search the www with expressions such as: Westcott Hort beliefs.
After seeing what these two Trinitarians were up to, note that "The Kingdom Interlinear Translation of the Greek Scriptures" states that its Greek text comes from Westcott and Hort.
More recent NT texts are available, such as Nestle/Aland.
They might have come from the original languages -- Aramaic, Hebrew, Greek, etc., but that does not guarantee the veracity of those sources, which requires the expertise of scholars in Lower (Textual) Criticism, such as Emanuel Tov.
Doug