Ideally, it is best to place the probe on a warm finger on the patient’s non-dominant hand so that the patient can still use their dominant hand without hindrance. For patients with decreased levels of consciousness (e.g. emerging from sedation), the middle finger is a good finger to place the probe because patients are less likely to scratch their face or eyes with this finger.  However, if you are unable to get a good reading on this finger, try the other fingers or the other hand until a good waveform is obtained.
References: Lifebox Pulse Oximetry Learning Module
Keywords: finger, placement, location