oštriti

1. (a knife, pencil) to sharpen; to hone
   Deda svake nedelje oštri noževe u dvorištu. — Grandpa sharpens the knives in the yard every Sunday.
   Dete je ceo čas oštrilo olovku. — The child spent the whole lesson sharpening a pencil.
2. (of mind, skills) to hone; to sharpen
   Igrajući šah, oštri um. — He sharpens his mind by playing chess.

The perfective is naoštriti (of blades) or izoštriti (of the mind, focus).