With the depletion of coal reserves all over the world, efforts are being made worldwide to utilize those deep underground coal seams, which are difficult to mine or are not feasible economically. Underground coal gasification (UCG) is a promising technology to achieve that objective and also to reduce the atmospheric pollution that is casued by surface gasification of the mined coal. Based on the previous research performed in this field, it is a well-known fact that the optimum quantity of water influx into coal seam can increase the hydrogen content of the product gas. In our earlier study, a novel method of utilizing humidified oxygen as the gasification agent was conducted for low ash coal gasification. The experimental results had shown the production of substantial quantity of hydrogen in the product gas. In the present study, the laboratory scale experiments are extended to high-ash Indian coals with increases in the absolute humidity of the feed oxygen by preheating the water (humidifying medium) at different temperatures in the range of 30-65 degrees C, with an interval of 5 degrees C. The composition of the product gas composition is estimated for the borehole gasification of high ash and low ash Indian coals with warm humidified oxygen as the feed stream. The optimum preheating temperature for the gasification of low ash and high ash coals is estimated as 45 and 30 degrees C, respectively. The average calorific value of the product gas is found as 239.8 kJ/mol for low-ash coals and 171.2 kJ/mol for high-ash coals. The average hydrogen content in the syngas during low-ash coal gasification is 26.9%, whereas, during high-ash coal gasification, it is 18.1%. The hydrogen gas is obtained as high as 33% for low-ash coals and 28% for high-ash coals.