To say Onam wishes in Malayalam, a common phrase is: "ഓണം ആശംസകൾ"
(pronounced: Onam Aashamsakal) This means "Happy Onam" or "Onam Greetings." Other heartfelt Onam wishes in Malayalam include:

• "ഹൃദയം നിറഞ്ഞ ഓണാശംസകൾ" (Hridayam Niranja Onashamsakal) — Heartfelt Onam wishes
• "സന്തോഷവും സമൃദ്ധിയും നിറഞ്ഞ ഓണം" (Santhoshavum Samridhiyum Niranja Onam) — Wishing a joyful and prosperous Onam
• "പൂക്കളത്തോളം നിറഞ്ഞ സന്തോഷങ്ങൾക്കായി" (Pookkalatholam Niranja Santhoshangalakai) — Wishes filled with happiness as much as the floral designs (pookalam)
• "ഓണം കൊണ്ടുവരട്ടെ സന്തോഷവും ഐശ്വര്യവും" (Onam Konduvaratte Santhoshavum Aishwaryavum) — May Onam bring happiness and prosperity

So, the straightforward and widely used wish is "ഓണം ആശംസകൾ" , and it can be enriched with more warm words depending on how formal or elaborate the wish is intended to be.