Are oblivion mods safe

Sort of, if you download them from credible sights like fileplanet or Nexus. but if you really want to be safe download OBMM (Oblivion Mod Manager) and learn to use it. OBMM can help you apply and remove Mods.
if you want to be really, really safe you can back up the data file under C:\Program Files\Bethesda Softworks\Oblivion and if you game stops working just delete the data file in the folder and replace it with your back up (Note: this will not effect your saved games).