(whistles innocently while kicking his 2.4 GB current save file under the sofa...) :)
Generally speaking, nhd files and the accompanying export package files are safe to delete. But any relationships with sims who live in those worlds will be gone as the sims who lived there will no longer ever have existed. A new world will spawn the next time you attempt travel there. I don't actually know if there are any special considerations for Oasis (probably not) as I don't play ITF.
But the best way to proceed, if you really want to dump the nhd file, is to give it a try on a copy of your game save. If unwanted or unexpected things happen, you can always dump it and return to where you were in gameplay.
