I am using 1769-OF8V Analog output module in 1769-L32E compact logix system.
Output is not generated from this analog output module. Status "OK" LED flashing green. how to rectify this problem ?
A quick google search turns up the manual for this card, which tells you that a flashing "OK" LED means external power failure. This card can be powered from the backplane, or from an external power source. If your OK light is flashing, it's obviously selected to external power, and there is no external power.
And here's the page showing the switch which selects bus power or external power. Although I would suggest that if this problem has only just presented, you should investigate the loss of external power further rather than just switching to bus power.