B737 Undifferentiated or Other Model
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 210
Aircraft Equipment Problem Critical
Flight Crew Diverted; Flight Crew Landed As Precaution
Aircraft
Aircraft Cabin Pressure warning horn sounded as Cabin Altitude warning light flashed while aircraft climbed through FL190. Aircrew applied Boldface/Boxed items. Aircraft diverted [to a nearby airport] for an uneventful straight-in approach/landing. Aircraft taxied to gate. Passengers were attended to and deplaned. Commendations to Center; Approach; the FO and FAs.
We immediately donned our masks and worked to establish Crew communication. The Captain initially had some issues hearing radio and intercom transitions but this was sorted out in a minute or two.  Prior to the event; there were no issues with the packs or ducts or pressurization controller. The only other light we observed was the OFF SCHED DESCENT Light.
B737 flight crew reported loss of cabin pressure climbing out of FL190. Flight diverted to a suitable nearby airport.