Look for strange eval(base64_decode(...)) strings or unfamiliar require functions at the top of these files.
The only permanent fix for a nulled plugin issue is to stop using them. Because Osclass is an open-source project that has gone through various iterations (like the Osclass Evolution fork), compatibility is key. osclass plugin nulled fix
Install a file-guard or security monitoring plugin to alert you of file changes. Look for strange eval(base64_decode(