5. ONGC-Imperial Energy: $2.8 billion

The Oil and Natural Gas Corp took control of Imperial Energy Plc for $2.8 billion, in January 2009, after an overwhelming 96.8 per cent of London-listed firm's total shareholders accepted its takeover offer.

Speaking about India's fifth largest M&A deal, ONGC chairman R S Sharma said the company owed the acquisition to government support, which has seen OVL in the past seven years increase its number of projects to 39 in 17 countries, from just a single project in Vietnam.