[open secret] {n.} Something that is supposed to be a secret […]

[open sesame] {n. phr.}, {literary} Immediate means of entrance, or unobstructed […]

[open shop] {n. phr.} A factory or firm that employs both […]

[open-door policy] {n. phr.} A policy that states that newcomers are […]

[opposite number] {n. phr.} A person occupying the same position as […]

[opposite sex] {n. phr.} The sex different from the one being […]

[optional origin] {n.} Stipulation in international commodities contract whereby the seller […]

[other fish to fry] {n. phr.}, {informal} Other things to do; […]

[out-of-pocket expenses] {n. phr.} Expenses one has to pay for oneself, […]

[outback] {n.} 1. The remote and uncultivated wilderness areas of Australia […]