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Opera Opera (trình duyệt web) Opera Software Opera Nova Bydgoszcz Opera Mini Opera GX (trình duyệt web) Operação Prato Opera Leśna Operatsiya Silent Storm Operation GreenhouseTài 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