The affix ṭhak comes in the sense of 'it exists' after the words 1. ojas 2. sahas and 3. abhas being in the Instrumental -3rd case in construction.,
Thus ओजसा वर्तते = औजसिकः शूरः 'a hero' lit. possessed with energy. So also:- 'a thief.' आम्भसिकः 'a fish.',