An octopus is just one thing beginning with the letter O that the kids could model out of playdough (and a fun thing to do, too, I think!) Can they think of more? Do you have any orange playdough to hand, perhaps?
Alphabet Begin With The Letter O Playdough Mat
A zip begins with the letter z - but what else? Get the kids to brainstorm some "z" words and then model them out of playdough. I feel a playdough zoo coming on ...