one

one one for sorrow; two for mirth; three for a wedding, four for a birth proverbial saying, mid 19th century, referring to the number of magpies seen at the same time.
one for the mouse, one for the crow, one to rot, one to grow proverbial saying, mid 19th century, traditionally used when sowing seed, and enumerating the ways in which some of the crop will be lost leaving a proportion to germinate.
one nail drives out another proverbial saying, mid 13th century, meaning like will counter like (compare fight fire with fire). The same idea is found in ancient Greek in Aristotle's Politics, ‘one nail knocks out another, according to the proverb.’
One Nation a nation not divided by social inequality; in Britain in the 1990s, especially regarded as the objective of a branch of or movement within the Conservative Party, seen as originating in the paternalistic form of Toryism advocated by Benjamin Disraeli.

In 1950 a group of Conservative MPs, then in opposition, published under the title One Nation a pamphlet asserting their view of the necessity of greater commitment by their party to the social services; these ideas had great influence when the party returned to government in the following year.

In the 1990s, One Nation returned to prominence in the debate between the right and left wings of the Conservative Party on the effect of the Thatcherite policies of the 1980s.
one size does not fit all an assertion of individual requirements; the saying is recorded from the early 17th century. Earlier versions of it exist, and are based on the metaphor of different size shoes for different feet, e.g. from J. Bridges Defence of the Government of the Church of England (1587), ‘Diverse feet have diverse lastes. The shooe that will serve one, may wring another.’
the one that got away traditional angler's description of a large fish that just eluded capture, from the comment ‘you should have seen the one that got away.’
one year's seeding makes seven years' weeding proverbial saying, late 19th century; the allusion is to the danger of allowing weeds to grow and seed themselves.
when one door shuts, another opens proverbial saying, late 16th century, meaning that as one possible course of action is closed off, another opportunity offers.

See also one Englishman can beat three Frenchmen, one law for the rich, one of our aircraft is missing, one step at a time, one swallow does not make a summer, one wedding brings another, one white foot, buy him.