How to convert .oga to WAV

[fricodam]

Hello all,
I am new to this player and to audio converting in general. I have .oga file that I want to use in FCPX. It should be either AIFF or WAV. i couldnt find AIFF option, and tried WAV. The file I have recieved is not usable and I am not able to open it in any program. Am I doing something wrong? Is there a tutorial for audio file convertion?
Thank you in advance.