Can I delete 'original' media if I keep the 'transcoded' media?

After just a few projects I've started eating up some serious HD space with original and transcoded media... Just wanted to know if you guys had insight on this.

Yes you can. Once your media has been transcoded your Event will link to the transcoded media instead of the original media. But we all know you cannot be careful enough. Sometimes you may have imported extra media in your Event such as stills or external footage that hasn't been transcoded. This footage will not be present in the Transcoded Media folder and it will be lost if you delete the contents of the Original Media folder.

So I would advise you to put the content of the Original Media folder to the trash but not empty the trash yet. Then you launch your FCPX project and you check if everything works as expected. If it does it is safe to empty the trash.

One other thing to check: if you have imported your media with the "copy files to Final Cut Events folder" UN-checked, your Original Media folder will not contain any actual original media but just aliases to the original media. You can see if it's just aliases by the little hooked arrow on the media files. In this case deleting the original Media folder of the Event will not result in extra drive space and you will have to go to the folder where the real original media are stored to delete them.

Also you want to move the Transcoded media ProRes into the original media folder before opening up FCPX.
There should be option to replace original with the ProRes clips.
Backing up tape less footage is a no brained anyways right?