The western wall of Osprey Reef, called North Horn Wall by Spirit of Freedom and Western Wall by Mike Ball, is a vertical face that runs south from North Horn and drops more than 1,000 m into the Coral Sea. Spirit of Freedom describes it as a 1,000 m vertical drift along the western wall with caves, overhangs and soft corals, and Mike Ball as a sheer wall with overhangs where dogtooth tuna, grey reef sharks and barracuda are visible. A ScubaBoard trip report divides North Horn into North Wall and West Wall sections.

The dive is typically run as a drift. The wall begins just below the reef edge at around 5 m and falls away beneath you into deep blue water. Shallow caves and ledges break the face near the top, while deeper overhangs are hung with soft corals and gorgonians. With no bottom in sight and visibility that often exceeds 30–40 m, the sensation is of flying along a cliff.

The open-ocean position brings in big animals. Manta rays, barracuda, dogtooth tuna and grey reef sharks are regular, and there is always a chance of silvertips or hammerheads out in the blue. Along the wall itself look for schooling anthias, butterflyfish, angelfish, triggerfish and the smaller life in the caves.

Because the western wall is the lee side of the atoll in the south-east trades, it is often the most reliable place to dive at Osprey when the wind is up. It demands good buoyancy and constant attention to depth, making it best for experienced divers comfortable in blue water.

Liveaboard only. Mike Ball's Spoilsport runs 4-night fly-dive trips (depart Cairns Thursday evening, cruise north overnight via the Ribbon Reefs, cross to Osprey, fly back from Lizard Island) and 5–7 night Coral Sea expeditions; Spirit of Freedom visits Osprey on its 4- and 7-night itineraries, with a Cairns–Lizard Island flight connection. The open-ocean crossing from the Ribbon Reefs to Osprey (about 100 km) is made overnight.

Allow 2–3 days at Osprey on a typical trip. The western wall is the lee side in the prevailing SE trades and is usually dived as a drift from a tender, starting near North Horn and drifting south.

Osprey is an isolated oceanic atoll about 350 km north-east of Cairns and roughly 100 km beyond the edge of the Great Barrier Reef, so every trip involves an overnight open-ocean crossing. Strong south-east trades (over about 25 knots, common June to August) make the crossing rough and can close the exposed eastern and southern walls; liveaboards then work the sheltered western wall and North Horn. Sep–Dec usually brings the lightest winds and visibility of 40 m or more.

Currents on the walls vary with tide and oceanic flow and can run strongly around the horns. Dec–Apr is cyclone season and trips can be cancelled or diverted to the Ribbon Reefs. As the lee side in SE trades, the western wall is usually the most diveable part of the atoll when it is windy.

Drop from the tender near North Horn, descend to 20–25 m on the wall and drift south with the wall on your left shoulder, checking caves and overhangs. Watch depth constantly as there is no bottom. Ascend gradually along the wall, deploy an SMB and complete the safety stop in the blue or over the reef edge.

Mike Ball Dive Expeditions (Spoilsport) – 4-night Osprey fly-dive, 5-night Coral Sea & Cod Hole and 7-night Coral Sea expeditions from Cairns, several a month; Spirit of Freedom – 4- and 7-night trips including Osprey, weekly. Mike Ball lists it as 'Western Wall'.