The font file called Oswald-RegularItalic.ttf was named incorrectly.

[stroobandt]

The font file called Oswald-RegularItalic.ttf was named incorrectly. It
should be named simply Oswald-Italic.ttf.
This is how other fonts are named. See for example:
http://www.fontsquirrel.com/fonts/source-sans-pro

The current situation causes serious problems at least with Ubuntu's
Font Manager: Oswald-Regular appears as an italic font, and an
additional Oswald-RegularRegular font is created containing only the
regular font style.

[modelworker]

This causes problems under Windows too, calls to the Regular font render as the italic.

[BGCDI]

Any chance this issue will be resolved?! My client would like to use the typeface for their corp branding and this is a deal-breaker.

[aronmoshe-m]

I was able to resolve this by opening Oswald Regular in FontForge. From there, go to Element > Font Info. Change the Family Name from 'Oswald Regular' to 'Oswald' and re-save the font. Once Oswald Regular is installing with the font family rather than as a separate font family, this seemed to fix the issue for me.