ex, opera

ex or e prep. with abl. 

• (1) in space , [from or out of]; 'ex equo pugnare', [to fight on horseback] (operating from it); 'ex adverso', [opposite]. 
• (2) in time, [since]; also [immediately after]; 'aliud ex alio', [one thing after another].
• (3) in other relations: to denote origin, [from, away from, out of, of]; 'ex animo', [heartily]; 'ex industria', [on purpose]; 'unus ex', [one of]; 'pocula ex auro', [gold cups]; to denote cause or occasion, [from, on account of, by reason of]; 'e vulnere mori', [to die of a wound]; to denote correspondence, [in accordance with]; 'ex re et ex tempore', [according to time and circumstance]; to denote advantage, e.g. 'e republica', [for the benefit of the state]; in gen. [in regard to]; 'ex parte', [in part].
  

opera -ae f. [trouble , pains, exertion]; 'operam dare', with dat., [to word hard at]; 'est operae pretium', [it is worth while]; 'opera mea', [thanks to me]. Transf. [time for work; work done]; in pl. [laborers, workmen], also [mobsters, gangsters].

... http://archives.nd.edu/latgramm.htm