This disclosure describes an Oxygen sensing patch that can be attached to the interior surface of packages for oxygen interrogation inside the package. The Oxygen sensing patch is integrated into the inside of the package during the packaging manufacturing process and, when used with a blue LED and fluorometer, will quantitatively report the amount of oxygen in the package. The Oxygen sensing patch is suitable for measuring oxygen in gas, such as headspace applications, and in liquid. Oxygen is sensed by measuring the phase shift of fluorescence of the fluorophore immobilized in the Oxygen sensing patch. The patch can be non-intrusively interrogated by a light source. The interrogation can be done using a phase fluorometer where oxygen level is desired, or inspected visually for color change using a hand held blue LED.