DUBAI, Jan 27 (Reuters) - Oman Electricity Transmission Company has tightened its price guidance for 10-year U.S. dollar-denominated bonds due to launch later on Wednesday to around 6%, a document from one of the banks on the deal showed. The company, owned 51% by Oman via the state-owned Electricity Holding Company (Nama) and 49% by China’s State Grid International Development Ltd, had given initial price guidance of 6.25%-6.375% for the bonds. It drew more than $3.1 billion in orders for the debt sale.