Mobile originated, or MO, describes a message or event that starts on the customer's phone and travels to the business: a reply, a tapped button, a STOP keyword.
What counts as MO
Anything the handset sends. In SMS that is a reply text, including the compliance keywords STOP and HELP. In RCS it also includes the structured events an RBM agent receives: a suggested reply tap, a suggested action tap with its postback data, a shared location, a delivery or read receipt, and a file or photo the customer sends. The platform delivers each one to the business's webhook as an inbound event.
Why MO matters for compliance and cost
MO traffic is how consent is exercised. An opt-in keyword, a YES to a double opt-in prompt and a STOP are all MO messages, and the sender has to process them immediately and automatically. On the billing side, US operators do not charge the business for inbound RCS events, while inbound SMS is usually billed at the same per-message rate as outbound; the pricing page lists both directions.
Mobile terminated is the other direction
The message the business sends to the phone is mobile terminated. A two-way conversation is simply an alternating series of MT and MO messages in one thread.