Can I change the value of the O & M cost for certain components?
If you are looking at a file where someone entered $500/yr for the O&M cost of some component, and you think $600/yr is a better number, and you are asking me if it's okay to change it, yes it's okay to change it. You should enter whatever numbers you think are most realistic. If you are looking at a sample file where I entered $500/yr for some component and you're wondering why I used that number, you will have to tell me what sample file and what component you are talking about because I can't tell from your question.
For most components (except for the generator) you enter the O&M cost in $/yr in the cost table. HOMER assumes that O&M cost occurs every year of the project lifetime, regardless of how that component is used. For example, if you enter $1000/yr for the O&M cost for one wind turbine, and $1500/yr for the O&M cost for three wind turbines, then if your system contains two wind turbines HOMER will calculate the O&M cost for those two turbines to be $1250/yr. That O&M cost applies every year, regardless of how much energy that turbine produces.
The generator is different in that you enter the O&M cost in dollars per operating hour, so if you enter $0.50/hr and the generator operates 6000 hours per year, HOMER will calculate the annual O&M for that generator to be $3000/yr.