To wish someone Onam in Malayalam, but written in English letters, you can say: "Onashamsakal"
(ഓണാശംസകൾ) This means "Onam wishes" or "Happy Onam." If you want to say a more complete greeting, you can say:
"Nalla Onam Ashamsakal"
(നല്ല ഓണം ആശംസകൾ)
which means "Good Onam wishes" or "Wishing you a happy Onam."

Example phrases to wish Onam in Malayalam (using English letters):

• Onashamsakal!
• Nalla Onam Ashamsakal!
• Ellavarkkum Onashamsakal! (Wishes to everyone on Onam)
• Sukhamaaya Onam Aashamsakal! (Wishing you a joyful Onam)

Feel free to use these warm greetings during the Onam festival to share joy and blessings!