The antonyms for the word "work loose" could be secure, tighten, fasten, fix or anchor. When an object is securely fastened or tightened, there is no possibility of becoming loose or detached from its position. Anchoring something properly ensures that the object is held in place and is not likely to move. Fixing a problem means that the issue has been resolved and there is no longer any room for error. Therefore, in contrast to "work loose," the above antonyms imply stability, reliability, and safety, and suggest that things are secure, firm, and in position.