Ryan O'Neal's Son -- Equipped with Monitoring Device

Ryan O'Neal's 26-year-old son Redmond got one hell of a parting gift during his sentencing last week for a probation violation -- a state-of-the-art GPS monitoring bracelet.

Redmond's probation was revoked in September -- after he allegedly snuck out of rehab and ordered drugs on a payphone -- and last week, a judge sent him back to the treatment facility ... with the monitoring device around his ankle.

According to law enforcement sources, the device sends back detailed information about Redmond's whereabouts every five minutes -- and if he steps outside his assigned area, an alarm goes off.

Redmond -- who's had a long history battling drugs -- pled no contest to drug and weapons charges back in August ... and was sentenced to a year in rehab.