MultiBoot OpenBSD/Debian

[Funkygoby]

Hello there!

Long description, then problem, then question.

Description:
My main OS is Debian oldstable for now on my main laptop and I used to teach myself OpenBSD with a spare dedicated latptop.
This second laptop as been lend to a friend who was in distress since here macbook had died.

Now, this laptop is back in my house and I can resume my learning.
I was planning to install Debian jessie for android dev and OpenBSD on his disk.

-First I installed Debian. I partitionned the disk like that:
sda1 50GB empty (for OBSD)
sda2 50GB for debian separated in 3 logicals partitions (/,swap and /home)
Installation went smoothly as I am seasonned with debian installation.

-Then, I proceed with OBSD installation (not my first time either but still newbie).
I may not remember exactly what I did but I am pretty sure I choose (e)dit at the partition step, didn't see the layout I was expecting (2 primary partitions), tried to build some layout anyway then cancelled.

At this point, I was expecting the layout to be still intact but I was wrong...

Problem:
On reboot, no os found. So I reinstalled debian 8 and I am waiting for more insight before retrying the OBSD install.

Question:
When does OpenBSD fdisk proceed? I though that any writing on disk wouldn't occur until I say so?