"OverSight constantly monitors a system, alerting a user whenever the internal microphone is activated, or the built-in webcam is accessed," the OverSight site reads. "And yes, while the webcam's LED will turn on whenever a session is initially started, new research has shown that malware can surreptitious piggyback into such existing sessions (FaceTime, Skype, Google Hangouts, etc.) and record both audio and video - without fear of detection."
Users get an on-screen notification anytime the camera or microphone is activated. That includes uses that are initiated by the user. So, an alert will pop up even when you're trying to FaceTime your mom.