There are a variety of different plagiarism checking tools online, both free and with fees. Two commonly used tools are Turnitin and iThenticate.
Whichever plagiarism checker you end up selecting, be sure to thoroughly check their terms of service as some free plagiarism checker websites sell uploaded documents to essay mills, an example of this is Viper Anti-Plagiarism Scanner.
OJS does not offer plagiarism checker software.