Oscar is a closed-source project, source code is not available.
RAR archives are not video files and as such will not be added to list of video files.
You can always try to use the highly experimental "archiver support" (see Preferences --> System --> use archiver support) in the latest beta to search for subtitles (to video files) in archives but I do not recommend it since calculating video file hashes from archives is quite slow at the moment.
Regards, Majky ...