Does saying "رأسي من رأسك حرام حتى تفعلي كذا" ("My head is forbidden from your head until you do such-and-such") amount to declaring the wife forbidden, or is it an oath?

The preponderant opinion is that as long as you did not intend ẓihār (a pronouncement making one's wife unlawful to oneself, akin to divorce) or divorce with this oath, then neither ẓihār nor divorce occurs if you break it. However, an expiation for an oath (kaffārat yamīn) becomes incumbent upon you.

Al-Nawawi said: If a man says to his wife, "You are unlawful to me," and he intended divorce, then it takes effect as a revocable divorce. If he intended ẓihār, then it is ẓihār. But if he intended to forbid her essence, or her private parts, or sexual intercourse with her, then she does not become unlawful to him, and an expiation for an oath becomes incumbent upon him.

Therefore, if you did not approach your wife until she did what you commanded her, then you fulfilled your oath, and there is no expiation upon you. But if you had intercourse with her before she did what you commanded her, then an expiation for an oath is incumbent upon you: feeding ten needy persons, or clothing them, or freeing a slave. And whoever cannot find [any of these], then fasting for three days.