Western names are not a common because they don't really exist. The names you gave -- John or Oscar -- are English names so belong to a subculture of the western world. You just assumed they are common because it's your culture. I bet you might find strange to see a João in a book, but it's western also (indeed the Portuguese version of the name).
A book set in Miami, for example, could have a lot of Juans and Joões in it but, even so, maybe somebody from other part of USA could think the names do not fit right since they are not used to such a proximity with Latin cultures.
That's to say sometimes you just can't attach too much with local context just because of that context. Local context, in a book, is not the real city's local context but the book's social context and, most important, your target readers social context. You can't expect somebody to know the context just because in the real world it's that way.
Basically, before to start a book, you need to know who are you writing for.
They first thing you need to have in mind is that, if you want to use local names in your book you need to contextualize the names if your target audience is not used to them. Context will make the users to leave the sound of the name behind and start feel like like they know the name.
A good example is Robin Hobb, who uses Adjectives for names. Fitz Farseer is son of Prince Chivalry Farseer who is married to Lady Patience.
Those names, at first, seem really strange but among the book you start to like then because you understand that they defines the characters, and because Robin contextualizes the why the six duchies favors adjectives as names instead real names.
In my current manuscript, I have two characters with foreign name: Dexter and Elton. I didn't chose by rolling dices. There's a why and I tried to make it real clear in the book how the characters end up with foreign names in a Brazilian context.
If you contextualize your book's background and names, you can use them as much as you want.