I reason that the answer is that “ought” logically implies a purpose, even though by convention (and for certain social reasons) that purpose is often unstated and even deliberately hidden*.
“Ought…in order to.”
The purpose could be absolutely anything, including “in order to get right with God,” “in order to get the approval of one’s parents,” or “in order to get other psychic/material/indirect/emotional/metaphysical/religious benefits.” Therefore, a conclusion of “ought” can only be completely objective if the stated purpose for that “ought” is completely objective.
*This is what confuses people to no end. One reason for the obfuscation is probably that there is good reason to hide the purpose of a moral imperative if you want no one to question it. People also dislike this fact because it can be easily twisted to convince people to do bad things, such as via the greatest happiness principle, but that is no reason to abandon logic.
With that out of the way, Hume’s challenge is easy enough:
Completely objective premise of “is”: Stealing is an act that deprives others of their property.
Completely objective conclusion of “ought”: I ought not steal. (Possible implied purposes/benefits: “in order to avoid revenge,” “to help society work better,” “to avoid getting arrested,” “to not feel guilty,” “to maintain my personal integrity, which I value,” “to not disappoint my parents,” etc. Note that insofar as each implied purpose is true, “I ought not steal” is a completely objective conclusion.)