Answer by Kjetil Joergensen for How can I allow one user to su to another without allowing root access?

Add to your /etc/sudoers something like

tom ALL=(oracle) ALL

Then user tom should be able to use sudo to run things as user oracle with the -u option, without letting tom

I.e. getting a shell as user oracle (well, given that your sudo is new enough to have the -i option).

sudo -u oracle -i