either will work.
Main Entry: oc·to·pus
Pronunciation: 'äk-t&-p&s, -"pus
Function: noun
Inflected Form(s): plural -pus·es or oc·to·pi /-"pI/
Etymology: New Latin Octopod-, Octopus, from Greek oktOpous
1 : any of a genus (Octopus) of cephalopod mollusks that have eight muscular arms equipped with two rows of suckers ; broadly : any octopod excepting the paper nautilus
2 : something that resembles an octopus especially in having many centrally directed branches