Which one is correct?
1- The OPS structure.
2- The OPSs structure.
3- The OPS's structure.
4- The OPS' structure.
OPS: A singular abbreviation.
In Proper Apostrophe Usage with Initialisms: CCS' or CCS's? the conclusion was that the trailing s is dropped when the thing's singular name ends in an s. Correct usage would then be #4, The OPS' structure.
I think there is a little more to it than that; for example my boss's house. I think the best guide is whether you actually pronounce the possessive as oh-pea-ess or oh-pea-ess-iz.
The most popular related question comes to the same conclusion.
Strunk & White's "Elements of Style" recommends always use 's.