Does this happen with other fonts? Do you by any chance have a number other than zero for Skew?
No, no number in Skew. Didn't happen with any other fonts either.
I'm not able to replicate the issue. Oswald Medium does not turn italic on my computer.
You could try removing Oswald Medium and installng from a different vendor to see if that works for you.
There is a free download at http://fontsfree.net/oswald-medium-font-download.html
I'm hoping that the issue is resolved for you.
If the above-mentioned solution or suggestion helped, please mark it correct.
If not, do let us know so that we can assist you further.
Thanks, Sheena! Solved for now.