Do these have the same meaning?
Oh is that so?
In informal use, they're pretty much equivalent. As a native speaker, I'd say they can both express skepticism or doubt:
"I caught a ten-foot trout yesterday."
"Oh, is that so?"
They can also both act as placeholders in conversation, phrases used to acknowledge that one party is listening to the other or to move the conversation along without making any substantive remark:
"I ran into your aunt Karen at the park yesterday.
The particular purpose depends in large part on the speaker's phrasing and intonation.
For what it's worth, "Is that so?" has enjoyed much greater prevalence, according to this ngram.
I see "Is that so" as confrontational, typically expressing disbelief or challenge rather than merely doubt. "Oh, really?" sometimes expresses doubt, and sometimes indifference. The specific meaning of either phrase strongly depends on context and manner.
Emphasis on the 'that' gives a negative tone, emphasis on the 'so' gives a curious tone. Same for oh really: emphasis on the 'really' is confronting, emphasis on the 'oh' is curious. This is only my experience with the terms in my culture, body language or general melody of the cultural language can change the desired effect of anything you say.
I see them as equal, with either having the potential of confrontation if delivered in such a manner. They could also demonstrate great interest and/or fascination. I see is that so as being used more by middle-aged to somewhat older folks, and really used by a middle-aged to younger people. The 'oh' is optional.