After the first reference the title should be abbreviated. Retain all the words up to and including the first noun. Whom God Hath Joined Together . It is not uncommon that an author has produced several works with similar titles. If this is the case, expand the abbreviated reference until you reach the first noun which makes the title you mean unambiguous.
Short titles are much better than using op.
This can be very confusing because it obliges the reader to trawl back through the earlier notes. Similarly, you should be very sparing in the use of ibid. Only do this when only one title is mentioned in both notes, so that there is no uncertainty about what the same work refers to. If in doubt, do not use ibid. Repeat the name.