Hey there!
Note that while \(\vec{AB}\) represents the same complex number as \(\vec{OC}\), you can't just use that to find \(\vec{OB}\), since \(\vec{OB}\) clearly does not equal \(\vec{OC}\) (one is the diagonal of the square, the other is a side!). What you've actually done is rotate \(\vec{OA}\) clockwise 90 degrees to give \(\vec{OC}\).
There are two ways to do this:
a) Consider that \(\vec{OB}=\vec{OA}+\vec{AB}\) - you've got both values; the first given in the question and the second you've just calculated as equal to \(\vec{OC}\). Add them up and you'll have \(\vec{OB}\).
b) Consider that \(\vec{AB}\) is a rotation anti-clockwise of \(\vec{AO}\) by 90 degrees. Recalling that \(\vec{AO}\) = \(-\vec{OA}\), we find that \(\vec{AO}=-2-i\). Rotating to find \(\vec{AB}\), the computation then becomes identical to the one above via addition of vectors.
There technically is also a 'cheat' way of doing it by inspection; it's a relatively simple square and some people might pick up that B is 1+3i straightaway, but there's no working out involved; for the most part this is only helpful for checking your answer.
Hope this helps