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Opera Opera Việt Nam Opera (trình duyệt web) Opera Nova Bydgoszcz Opera Software Operation Mincemeat (phim) Opera Mini Opera GX (trình duyệt web) Operação Prato Opera LeśnaTài liệu tham khảo
WikiPedia: Opera http://www.opera-opera.com.au/archives.htm http://www.opera-opera.com.au/plotind.htm http://www.american.com/archive/2007/july-august-m... http://www.britannica.com/EBchecked/topic/348741/L... http://www.britannica.com/EBchecked/topic/429776/o... http://books.google.com/books?id=fovWU9Prkj0C&lpg=... http://www.historicopera.com http://operabase.com http://www.stageagent.com/browse/showtype/opera http://opera.stanford.edu/main.html