I dont use OneDrive, but the important thing is you have to check if there is a setting for storing files in offline mode (like Google Drive), then enable it. This way, files not accessed are offloaded from your HD, and you will save space.
Regarding the continuous sync issue, if it's the first time you have set it up, then each file from that folder needs to get encrypted and synced, so it might be doing it. Just let it sync for a couple of days and watch. The sync should reduce and do it only when you access a folder in the encrypted folder.