How much does an O-1 make a month?

It depends on years of service. I have posted links for the 2010 & 2011 military pay charts. In 2010, an O-1 starts at $2745 per month (with less than 2 years service). An O-1 maxes out at $3455 per month after 3 years, with the exception of the O-1E (prior enlisted service), which has its own chart: $3453/month at 4 years up to $4289/month at 16 years. These are basic pay rates and do not include any allowance for quarters (housing) or rations (meals), that might be available.