In the above, do you say that "You ought to do X." is exactly equivalent to the command"Do X!", and "I ought to do X." means "I will do X on the first opportunity and not by accident." ?

To the first one, yes, but they have different connotations.

To the second one, sort of. "I" can get fuzzy here. I have akrasia problems. I should do my work, but I will not do it for a while. If you cut out a sufficiently small portion of my mind then this portion doesn't have the opportunity to do my work until it actually does my work, because the rest of my mind is preventing it.

Furthermore I am thinking about them more internally. "should" isn't part of predicting actions, its part of choosing them.

Ought we base the definition of "ought" on a pretty complicated notion of rationality?

It doesn't seem complicated to me. Certainly simpler than lukeprog's definitions.

These issues are ones that should be cleared up by the discussion post I'm going to write in a second.