You can't "convert" files. They are encoded to disk specific to the file type, so you have to reopen and resave as the new file type. However, if you saved the file and then just typed a new extension in error, simply retype the correct extension.
(I first thought .omg was a joke, but it turns out it's a legitimate extension used by O&O DiskImage, a disk image creation and backup application).
If you started from PSD files those files should still exist. I you just add layers and never save a psd you never had psd file you only had open layered documents. How were you able to save disk image OMG file from Photoshop? Photoshop does not seem to have support for OMG file extension.
Do you have some Photoshop add on you not telling us about?