Looks like there's a back-door security issue. Update your plug-in with the latest version. http://searchengineland.com/update-one-seo-pack-wordpress-plugin-now-193204
Glad I don't use that plugin for SEO, thank you for informing! I use Yoast SEO. And a note for everyone, please keep all of the plugins updated at all time even if you don't use them and if you don't use some of plugins in your blogs then remove them as they will load off your server weight and more safer that way.