1)
What is the difference between Oyvai and Kamai?
1.
Malbim (Tehilim 18:49): Oyvai are those who covertly seek my harm. Kamai are those who openly fight me.
2)
Who is "Ish Chamasim"?
1.
Malbim (Tehilim 18:49): It is Sha'ul, like it says "mi'Kaf Kol Oyvav umi'Kaf Sha'ul" (verse 1).