Established by a coalition of syndicalists and socialists, in the wake of a strike wave that followed the 1910 revolution, the UON was the first trade union centre to unite workers across different industries from throughout the country.

[3] In 1909, anarcho-syndicalists in Lisbon held a congress of trade unions and cooperatives, during which they demanded the eight-hour day, established a General Confederation of Labour (CGT) and set themselves the goal of bringing the production of essential goods under workers' control.

[11] Due to its internal dissonance, the UON initially struggled to accomplish its tasks, with its newspaper União Operária only publishing two issues.

[15] UON member Manuel Joaquim de Sousa [pt] also attended an anti-militarist congress in Galicia, where syndicalists from around Europe discussed an international general strike to end the war.

That same month, the union's construction workers organised looting in the Campo de Ourique neighbourhood of Lisbon, culminating in a riot that ended in a shoot-out with the police.

[20] In the spring of 1917, several workers' conferences were held in Lisbon and Porto, which brought together 176 trade unions, 4 industrial federations and numerous left-wing newspapers and cooperatives.

[24] In May 1917, Machado declared martial law and suspended the constitution in Lisbon, which provoked further protests and riots, leading to shoot-outs with the police and mass arrests.

[26] The general strike was successful,[26] managing to secure the return of the offices to the federation, the release of arrested construction workers[27] and a 50% wage increase.

[29] By 1918, the UON had been reorganised and reached the peak of its organisational capacity, as the outbreak of the Russian Revolution and the establishment of a dictatorship by Sidónio Pais raised revolutionary sentiments among Portuguese workers.

Júlio Botelho Moniz attempted to secure the UON's support for this measure by proposing that the unions take over the sale of food, but the workers refused to form such a pact with the dictatorship.

[...] However, as long as the necessary revolution does not take place, the workers have only one way of ensuring their existence: the permanent struggle for higher wages.In May 1918, the UON began planning for a nationwide general strike with the goal of opposing the rising cost of living.

[31] Large amounts of southern Portugal were brought to a standstill, while in Lisbon, the strike was supported by printers, furniture makers and construction workers.

[26] On 23 February 1919, the UON published the first issue of its periodical A Batalha, which grew in readership to achieve the third largest circulation of a Portuguese newspaper.