The Nam Ou (Ou River) joins the Mekong beneath a dramatic karst formation that, from some distance south, looks like a vast green eagle taking off. Facing it is the village of Ban Pak Ou, where a handful of river-front restaurants gaze out across the Mekong. On the other side (there's no bridge) are two famous caves cut into the limestone cliff. Both are crammed with Buddha images of various styles and sizes. A few steps from the river, the lower 'cave' is actually more of an overhang. A group of Buddhas pose perfectly as silhouettes against the grand riverine backdrop. To reach the upper cave, follow stairs to the left and climb for five sweaty minutes. This one is 50m-deep behind an old carved-wooden portal. If you didn't bring a torch (flashlight), you can borrow one for a suitable donation from a desk at the front.
Appealing as the scene may be, the caves themselves are less of an attraction for many visitors than the Mekong boat trip from Luang Prabang to get here. Be aware that you'll see the exact same stretch of river should you do a Luang Prabang–Pak Beng or Luang Prabang–Nong Khiaw boat trip. Between January and April, villagers along the Mekong sandbanks en route pan for gold using large wooden platters.
Whether by road or by boat, most visitors en route to Pak Ou stop at the 'Lao Lao Village' Ban Xang Hay where the narrow footpath-streets behind the very attractive (if mostly new) wat are full of weavers' looms, colourful fabric stalls and a few stills producing the wide range of liquors sold.
Luang Prabang's longboat office sells return boat tickets to Pak Ou (per person/boat 60,000/400,000K return) taking two hours upstream, 1¼ hours back and allowing around an hour at the caves plus 20 minutes at Ban Xang Hay. Departures are most numerous around 8.30am but generally continue all morning. Travel agencies and guesthouses sell the same tickets for a little more, often including a tuk-tuk transfer.
An alternative is to go by road to Ban Pak Ou (30km, around 150,000K return for a tuk-tuk) then take a motor-canoe across the river (20,000K return). Ban Pak Ou is 10km down a decent unpaved road that turns off Rte 13 near Km 405.