When I want to delete a photo or video from one drive, will it delete it from my phone? I just want to free up space and I don't even have my phone linked but somehow all the photos and videos are uploading anyway.
Windows Phone 8(.1)
Just like others said before, all changes are being synced, including deletion. There's nothing you can do :(
Windows 10 Mobile
There is hope! :D
Unfortunately I can't confirm this, but you can try by taking a non-important test photo, waiting for it to sync and then trying to test my solution on this photo.
Solution: get into photo's options and then unlink the local version of the photo from the OneDrive version (after this two copies of your photo will show up in your gallery - first local, second from OneDrive). After unlinking try removing the photo from OneDrive (remeber to remove it completely, I mean: even from OneDrive's recycle bin, it can't exist anymore on OneDrive). Then watch closely the gallery on your phone (refresh, if you have to). If the OneDrive copy suddenly vanishes and local copy still exists, then my solution is working! :)
You need to disable synchronization your OneDrive with your photos. Then you'll can delete photos from your OneDrive.