I got that frustrating "OC" code on my Vista 10. Here's how I ultimately ma naged to resolve the issue.
After coming home from a trip, I opened my door and entered my code on the key pad as usual ........and nothing happened. I did it over and over again but after 1-minute the alarm went off. The security monitoring firm called me but we could not get the alarm to stop blasting away. So with the rep s till on the line, I (1) got a screw driver and disconnected the Ademco AC p ower transformer from its wall socket and (2) opened up the Ademco control panel in the closet and disconnected the red wire from the battery. This st opped everything,including the blasting loud alarm. However, when I reconne cted up both AC and battery power all I got on my Vista 10's key pad was an "OC" code message. Hunting through the manual, looking at other web posts, and attempting different entries on the key pad did nothing. So I disconne cted BOTH the battery and AC power transformer, and then reconnected ONLY t he AC power transformer. This solved the problem!!!! Initially, I got a " dl" code on the key pad display, but it went off within about 1-minute and everything was back to normal.
Apparently my battery had run down to the point that it was causing an open circuit (OC) condition to appear to exist. Don't know why. Don't know how. Placing the system on only AC power apparently allowed the Vista 10 to re set on its own. I have now installed a new battery and it's fine too. (P.S. You may find you are able to just reconnect up your current battery after your system has reset using only AC power. However, if your battery is more than 3 years old you should probably replace it.)