How do malayalis greet each other for onam ?

if you are talking to someone , you can say onashamsakal.. 
if you are writing to someone for onam greetings, you can say hridayam niranja onashamsakal 
if you are over phone for onam wishes, you can very well say Happy Onam !!!!


Malayalis greet for onam by saying "onasamsakal"or "ponnonasasakal" 

in poetic language.; 

"hridayam niranja onasamsakal" 
"nanma niranja ponnonam asamsikkunnu" 
"samridhiyude ponnonam asamsikkunnu" etc... 

Onam wishes in malayalam

Maveli nadu vaneedum kalam, manusharellarum onnupole”

"nanma niranja ponnonam asamsikkunnu"-Wishing you Happy Onam

"hridayam niranja onasamsakal"-Heartly Onam Wishes

"samridhiyude ponnonam asamsikkunnu"

"ellaavarkkum ente hrdayam niranja onaashamsakal" - my hearty onam greetings to all.

"santhoshathindeeyum, sambalsamrithiyudeeyum, nirakudamaaya eee naalil, ningalkkellaavarkkum ente ponnooonaashamsakal" - on this overflowing day of happiness and prosperity, i wish u all a wonderfull onam.

'Onashamsakal..................' - onam wishes