On the other hand,
Yes, but . . .
You may be right, but . . .
I may be wrong, but . . .
Correct me if I’m wrong, but . . .
On the contrary,
(Be careful with this one!)
It appears to be a direct negation of what was just stated,
but it can actually be an emphatic reaffirmation of one’s own opinion.
For example: It’s not hot. On the contrary, it’s cold.
("Not hot" and "cold" mean the same thing.)