The main modules are Order Mgt, Receivables and cash management. Although other management needs inventory as well but this is not main part of the O2C modules
1. Inventory - selling items
2. Price list - Modifiers - pricing the item
3. Customer Master - Shipping to the customer, who will pay for the item
4. AR - Receivables - Invoice the customer for the shipped items
Shipping execution can be stand alone module or Default comes with Order mgnt
It all depends on business process.
Suppose business feel Order taken by some Order taking team and picking, packing,staging done by some other team
Then these can be seperated.
Just to add to the above answer, in Order management basically booking of order takes place but in Shipping Execution module, items which are in warehouse are released to staging area, packed, and then shipped.