There's no guarantee that a work won't be plagiarized. However, once you have a version that you're willing to make public, deposit it in an OA repository. That will give the work an authoritative public time-stamp. If you're plagiarized, you can use it to prove your priority.
Don't buy the myth that OA somehow authorizes plagiarism. For example, a CC-BY license requires attribution (that's what the "BY" means). All other other CC licenses also require attribution as well. OA licenses are enforceable.
Don't buy the myth that OA somehow invites plagiarism. As I put it in my book < http://bit.ly/oa-book > (pp. 23-24), not all plagiarists are smart, but the smart ones will not steal from a work indexed in every search engine. OA makes plagiarism easier to detect and expose. In that sense, OA deters plagiarism.