Mobile Originate Billing (MO Billing) refers to messages which are billed to the account when sent from the handset. This is not a subscription type service (MT Billing model), the handset must send the SMS message in order for the billing to take place. 

Generally the service content provider will respond with an MT message delivering content back to the handset. Usually this MT message will not have any further charges associated with it. 

Also see: MOMT and MT Billing.