To state that \( \triangle OPQ \cong \triangle RST \) by the SSS (Side-Side-Side) Congruence Theorem, you need to establish that all three sides of one triangle are congruent to the corresponding three sides of the other triangle.
Since two corresponding sides are already marked as congruent, you need to find the additional side that corresponds with these two that is also congruent.
The correct additional piece of information that is needed would be:
OP¯¯¯¯¯¯¯¯≅RT¯¯¯¯¯¯¯
This implies that the third corresponding sides are also congruent, allowing the application of the SSS Congruence Theorem to conclude that the triangles are congruent.