Octopus defense is primary avoidance and flight but they can bite with their sharp beak, have potent venom, in some cases enough to insure or kill a human, and are very strong. When threatened the Octopuses first reaction is normally to release Ink and initiate flight.
The ability to change skin color and mimic surrounding is a great avoidance/protection skill which they utilize in self preservation as well.
In addition to using their ink sacs and camouflage via the specialized skin cells, some octopuses can autotomise their limbs which will grow back. In the case of the Mimic Octopus there is also a fourth defensive option: mimicking more dangerous animals!