What is the object of the preposition in sentence? Scrooge will walk alone From his office.

The preposition of that sentence is from, and the object of the preposition is the next noun. So the object of the preposition is office.

In the sentence "aboard the ship" the preposition is aboard. The object of the preposition is what the preposition is referring to. For example in "aboard the ship" the preposition (aboard) is talking about the ship, so the OOP

(object of the preposition) is the ship.