The Indian Order of Merit was introduced under the Honourable East India Company in 1837. It would therefore have been presented under the authority of the Governor-General of India until 1858 when the territories and assets of the East India Company came under the direct control of the British Crown following the 1857 Indian Rebellion.
The Act of Parliament that transferred the territories and assets of the East India Company to the direct control of the British Crown was the Government of India Act 1858. The Act vested the power to appoint the Governor-General in the sovereign. The roles and responsibilities remained largely unchanged, so the award would have continued to be presented under the authority of the Governor-General who would then have been acting in the name of the British monarch.
Note that from 1858, the Governor-General was commonly styled the Viceroy and Governor-General of India, or simply the Viceroy of India. However, the 1858 Governor of India Act did not use the term 'Viceroy', and neither did the Warrants that appointed later Governors-General. For example, the London Gazette of 29 August 1947 records that:
The KING has been pleased to appoint Rear Admiral the Rt. Hon. the Viscount Mountbatten of Burma, K.G., G.C.S.I, G.C.I.E., G.C.V.O., K.C.B., D.S.O., to be Governor-General of India.
As you can see, despite being popularly known as "The Last Viceroy of India", there is actually no mention of 'Viceroy' in his official title.