There is a difference of opinion on this issue. Most employment law attorneys take the position it is legal to discriminate based on criminal law convictions. However, the ACLU does not agree.
The ACLU (American Civil Liberties Union) has taken the position that, "The EEOC and some courts have held that employers' bans on hiring people with convictions or arrests can violate Title VII of the federal Civil Rights Act of 1964 in the absence of a "business necessity," because such bans have a disproportionate impact on people of color."
Here is a brief discussion of Ohio employment laws:
In Ohio, "It is unlawful for an employer to discriminate based on race, color, religion, sex, national origin, disability, age, or ancestry of any person to discharge without just cause, to refuse to hire, or otherwise to discriminate against that person with respect to hire, tenure, terms, conditions, or privileges of employment, or any matter directly or indirectly related to employment." Criminal history is not a protected class.
Here is information from the EEOC on impermissible discrimination http://www.eeoc.gov/facts/qanda.html. Ohio also prohibits discrimination based on pregnancy and disability. Here is a link to the Ohio Civil Rights Commission: http://crc.ohio.gov/
If the person you decided not to hire was a minority, he may have convinced the ACLU or another rights organization that the reason was not only because of the conviction, but also discriminatory.
I am not sure what type of position he was interviewed for, but you may have a good business reason for not hiring him. For example, if he was being considered for a job as a driver and had a felony DUI conviction, that would be a good reason not to hire him.
By the way, if you carry CGL insurance, you may have insurance coverage for this claim against your company. You should contact your insurance agent and ask.