Geographically, the islands are volcanic remnants separated by the narrow, 137-meter-wide (449-foot)) Āsaga Strait, composed of shallow-water coral reef.

Before 1970, people crossed between the two islands by waiting until low tide and then wading across the shallow water of the strait.

[3] The main village of Ofu is located on the western shore, protected behind an offshore islet (eroded tuff cone) known as Nuʻutele Island.

The marine site has been part of long term research and study on coral reefs and global climate change.

The island is also home to the Samoa Flying-fox (Pteropus samoensis), a species of bat threatened by habitat loss.

"[5] Olosega Island is a remnant of the Sili shield volcano, the caldera of which may lie submerged off the north shore.

[6] Sili, situated on the northwestern-facing shore, consists of but one standing inhabited residence after much of the village was destroyed by cyclones (and subsequently abandoned).