According to the Collins Thesaurus, the most likely antonym for "freedom" is "slavery." However, because the word "freedom" has a complex and nuanced meaning, concepts like "dependence" or "limitation" may also be considered its opposite.
Dictionary.com defines "freedom" as a person's state of being free from some hindrance such as chains, regulation, prison or commitment. Slavery, in contrast, is a state of being owned. While one may call himself a slave to the state, that is rarely a literal statement. Rather, a "slave to the state" indicates someone who feels over-regulated or overtaxed by the government in some way. A person may also lack freedom due to his own good or bad choices, such as marriage or the decision to take a job. In these situations, the antonyms "limitation" or "suppression" are likely better matches.
In other cases, a person's situation creates a lack of freedom. A person who is paralyzed, for instance, has a limitation and is likely dependent upon others in order to live. A child is by definition dependent upon a parent. In all these cases, the opposite of "freedom" is "dependence," not "slavery." In society, William A. Galston observes,"freedom for the pike is death for the minnow." In other words, freedom for those with less power may be relative to those who have more power. For this reason, referring to a "free society" is nearly meaningless, as freedom must be curtailed for some in order to ensure it for others.