The bus station is located at the edge of town, 30 minute walk from the boat station, or you can get a local sawngthaw (mini bus with two rows of seats). You will probably have to charter one if you have not arrived by boat, as they tend to run when the boats off-load there passengers. From the bus station you can get a minibus to Luang Prabang or an old fashioned bus to Odoumxai (around 10 am). There are times posted but they will normally wait until the boats arrive so they gave a better chance of filling the bus.
From the small bus station you must walk about 30 minutes to the boat landing where the passenger boat leaves for Muang Ngoi. The ticket office is at the shack at the top of the steps, not far from the bridge.