I need to use a proxy in order to use Wifi at the office.
For 'Tor browser bundle' (on my laptop) I know where to write the Wifi proxy settings,
but where do I enter them when using orbot? (on samsung GT-s7102).
I believe what you're looking for is near the bottom of the settings screen in Orbot. There is a section for "Outbound network Proxy (Optional)" that lets you set an HTTP, HTTPS, Socks4, or Socks5 proxy.