GNU Octave is contained in the SlackBuilds repository, which contains building receipts for several software packages. The SlackBuilds Howto page describes in detail, how to use such a receipt.
The receipt for Octave has dependencies, which aren't automatically resolved and are listed in the README file of the Octave receipt.
Those dependencies are:
Note that there are further dependencies which are optional, e.g., GraphicsMagick. Another more general list of dependencies is given in the Building wiki page.