I have been thinking about this skill and unless you have a small magazine…or are constantly willing to reload, I think a few points can possibly be skimmed from this skill to fill up something like FollowThrough…which I LOVE!
But then I got to thinking about the DPUH…
How does One Shot One Kill work with a DPUH
Ammo cost is six shots…but you literally have 14 pellets out there.
Do they ALL get the bonus??
All 14 pellets of your first shot will get the bonus but iirc not the splash damage.
And why not spec into both skills? Unless you are maining a Vladof Pistol or an AR, OSOK is a good skill to spec into. Sure its damage contribution may not be as dramatic as it is on snipers and jakobs shotties, but any weapon capable of killing in several shots will benefit (Killer is very powerful, hence you can make a habit of reloading all the time 
Thanks!!
Oh I would put some points into it…probably at least 3-4…
I just really want to spec FollowThrough at 5/5
The gun damage bonus is nice but the movement speed bonus is just incredible.
With a Shadow Stalker mod…FollowThrough at 10/10 is just a hoot!
You are almost untouchable.
Also all unlisted pellet get the bonus too, Bekah…Pimpernal…etc. So it is worth it to put as much points in it as you can. 60% Additive damage adds up when you consider splash, unlisted pellet…then splash of those unlisted pellets…
If you have points in Fast Hands and Killer then just reload after a kill, amazing reload speed so you can always keep OSOK Bonus.
I am actually pretty good at early reloading so I think…with you guy’s advice…
I will leave OSOK at 5/5
THANKS!!