If you want the same technology as medical oximeters from the convenience of your smartphone, then download the Instant Heart Rate: Heart Rate & Pulse Monitor app. Available for free on Android devices, this app maintains a 4.3 rating.
Instant Heart Rate is easy to use with a finger and the capabilities of your smartphone. You can measure your heart rate and many other things in under 10 seconds. It even comes with a workout monitoring feature for when you exercise.