Make a huge O out of playdough and decorate it with smaller Os! It's a fun way to learn the letters of the alphabet. You could laminate the whole set of playdough mats and bring them out for your "letter of the week" activities.
Alphabet Decorate the Letter O Playdough Mat Upper Case
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?