I don't think there's a big difference at all between Ventress, Kenobi, Savage and Maul in swordfighting.
Indeed, there isn't. It's a matter of versatility.
Opress even before Dooku's training was fast enough to kill a master and padawan without lightsaber. His unnatural strength was a surprice element against his opponents.
Strictly saber skill was, also, alright, he was able to block all saber attacks even from Jarkai Kenobi and Sidious.
But he lacked maneuverability and was particularly vulnerable to unarmed combat techniques.
He disarmes Ventress, yet, she landed several kicks and punches after.
He forced Adi Galia onto defensive, yet, she managed to kick him in the knee.
When Kenobi was attacking him, he had no problem countering saber attacks, yet, was helpless against kicks.
Maul used Makashi style to counter Opress' strength and applied a joint lock technique - a move Opress simply did not expect.
Sidious kept landing kicks past his defence. When left alone, wise choice would be to fight defensively and try to survive until Maul rejoins but Opress on opposite kept attacking Sidious. When oponent fights offensively, it is much easier to expose weaknesses in his defence.