The narrator's Mother's Father, whose family name was Newpert married a daughter of the Oh family.
Maybe, but then again maybe not.
The maternal grandfather's name was Newpert, and the mother's name was Oh. That could mean one of several things:
(1) the mother's maiden name was Newpert and she married someone named Oh;
(2) the mother's name was Newpert but she had it legally changed to Oh;
(3) the maternal grandmother's name was Oh and this name was given to the mother;
(4) the mother's name was Oh before she was adopted by Newpert, and her name wasn't changed;
(5) the mother's maiden name was Newpert and she was first married to someone named Oh before she married someone else; or
(6) the author made a mistake.
From the original "excerption"
(which is a mess), it's not easy to tell. I lean toward the last explanation: the author made a mistake.
The narrator's maiden name would have been Newpert, before marrying into the Ashenden family.
No, that's not possible. If the narrator is a woman (something we cannot tell), her maiden name would, under normal circumstances, be that of her father -- presumably Ashenden. The family tree would look something like this:
The narrator is the product of the union of Oh and the unknown male, presumably Ashenden.