Proto: (c white and) at b plus at a equals oiotoshi?
Robert Pauli: Only if at b is urgent, e.g.
Bill: To understand oiotoshi it helps to understand the related phrase, connect and die. In the original diagram the Black stones are already dead. For that reason they cannot die. Connecting does not change that.
In the second diagram the stones are alive (on the standard assumption that the external Black stones are alive). They may be captured, however, if Black connects them to the stones. Connect and die. That's why is oiotoshi.