Overblows have a natural tendency to be slightly flat, so in order to be able to use them they have to be bent up.
Play the overblow, softly without force, and try to bend it up slowly. As you bend up the overblow further, the draw reed will vibrate further from the plate, more air is lost and therefore more air will be needed to keep the overblow sounding.
While doing this try to concentrate on keeping the blow reed choked and try not to put tension in muscles that have no influence on the pitch of the overblow. That means; keep the cheeks, lips and tip of the tongue relaxed.
Once you feel you have a good control over the pitch of the overblow practice playing it in the right pitch from the start.
Use a piano, or other sound source to check your intonation.
I find that playing tunes I have known from when I was very young are good for practicing my intonation. When I play songs I learned to sing as a child I am very aware of the notes I want to play and how they should sound.
Don't think of the notes as "hole 5 overblow" but instead hear the note you want to play in your mind before you play it.
When you have been practicing your overblows a lot, you may have gotten used to the flatness of the notes, to stop this from happening try to record yourself regularly and make sure your intonation is correct.