Birdy the Mighty (I)

While pursuing a criminal, the pretty but reckless interplanetary police officer Birdy Cephon Altera
accidentally kills Tsutomu, an Earthling high school student.
To make amends, Birdy is forced to share her own body with Tsutomu's spirit so that he can continue his own life.

At first, Tsutomu knows nothing about his situation,
but he soon discovers that his body is starting to take certain actions without his mind ordering it.
In times of action, Birdy takes full control of their shared body, transforming it back into her original female form.

NOTE: Volume 1 came out 7/1996 and was never finished.
A second series was published under the manga title ＶＳ版　鉄腕バーディー for 2 volumes.
This second series came out 12/1996. (Source: MangaUpdates)

Link:
AnimeNewsNetwork (OAV)