Could you why there would be an O&M cost for the grid?
HOMER reports the cost of buying grid power as the “grid O&M cost”. If the system you are modeling consists of a wind turbine and the grid supplying an electric load, the system probably meets the load partly with wind power and partly with grid power. The cost of buying that grid power gets reported as grid O&M cost. The Grid tab of the Simulation Results window should give a more detailed breakdown of that number.