Yes, they can be opened from the outside, happened even during a deicing before take off on one of ours by the deicing boom.
The overwing emergency exits lock when:
• three of the four Entry/Service doors are closed and
• either engine is running and
• the airplane air/ground logic indicates that the airplane is in the air or both thrust levers are advanced.
The overwing emergency exits unlock when any one of the above conditions is not met or DC power is lost.
From my (slightly older) FCOM.