I have a RZ4619 zero turn mower and the oil now light came on and it stopped running.

check safety switches, seat, brake, and deck.,

Sounds like a spindle bearing out

Check your drive belt. May have broke or come off

These units use Hydraulic pumps for motivation. If the unit stopped all at one time, I suspect a drive belt came off.
For both pumps to fail at the same time would be very odd. Check the v-belt driving the wheel pumps.

There is not a reset button. The gauge should blink for about 7-8 hours then stop. If yours continues to blink, it may be bad.

Check your oil level. Most mowers won't start if the oil level is too low.

If the warning light says "chg oil" the meter is telling you it is time to change the oil. The meter will automatically re-set after 6-8 hours of run time. It cannot be manually re-set.

Could be a bad sensor.

No, this is a micro switch issue, to prove this find the single black lead coming from the ignition unit ( cut out wire ) and diss connect it and try again, be aware it will not switch off at the ignition switch with this wire not connected.

the safety switch is bad! you can unplug the switch, then try it!
(to test)