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Results found in several dictionaries: Collins, Collaborative Dictionary
Omán   sm Oman

Collaborative Dictionary     Spanish-English
Mar de Omán pref. Arabian Sea Reverso General Dictionary 
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'omán' also found in translations in English-Spanish dictionary
Arabian Sea n. el Mar de Omán Reverso General Dictionary 

"Collins Spanish Dictionary 8th edition published in 2005 © William Collins Sons & Co Ltd 1971, 1988 © HarperCollins Publishers 1992, 1993, 1996, 1997, 2000, 2003, 2005"