About openbsd partitioning ....

The default OpenBSD partitioning is not mandatory, you can use a single partition if you want.
In fact, on an EFI system, you should have at least 2: the EFI system partition and OpenBSD's 4.2BSD partition.
Note that you can't put those partitions where you want : the EFI system partition has to use letter 'e', the 4.2BSD partition 'a' and 'c' is used to map the full disk.
The simplest way to get things done is probably to edit the default layout, delete all the partitions you don't want and modify partition 'a' to use the whole available space.

I haven't been able to install OpenBSD on an EFI system so far, even under QEMU, I hope you'll have more luck.

The default partitioning worked fine on my UEFI computer so at least it may work.

20-100 the EFI system partition has to use letter 'e'

On my OpenBSD install e is /usr (a is /; bis swap; d is /home and f is an "outside" Linux ext2 partition which is used to share data between OSes)

20-100 I haven't been able to install OpenBSD on an EFI system so far

Worked just fine for me. Did you copy the BOOT*.EFI files from here into OpenBSD's EFI System Partition?

I've just made a quick test with OpenBSD 6.8 and I confirm the installation ISO fails to boot in UEFI mode.
However, as NetBSD 9.x issues with UEFI have been addressed in CURRENT, one can expect a future version of OpenBSD to work fine too.

For the 'e' slice, you're right, I've got mixed up with NetBSD, sorry.

Lucky you!

I have several machines on which I make installation tests from time to time: a Lenovo Ideapad 120S (2017), an Intel NUC6CAYH (2016) and a Dell R610 (2010-2012).

The NUC6 is interesting because it supports both UEFI and legacy boot at the same time. Some years ago, I had installed Void Linux on it and it worked fine. At some point, I reinstalled it using a more recent Void ISO and it failed. With further testing, I realized that whichever OS I tried to install failed when both UEFI and legacy boot were enabled.

However, with just UEFI enabled, I could install any Linux distribution or FreeBSD, but neither NetBSD, nor OpenBSD (nor OpenIndiana and Tribblix, by the way). In legacy boot mode, I could install anything.

The R610 supports either UEFI or legacy boot, but not both modes simultaneously. However, the result was the same: Linux or FreeBSD could be installed in UEFI mode, but neither NetBSD, nor OpenBSD.

I have updated the NUC6 and the R610 firmware to their latest available versions but it made no difference.

I have recently tested NetBSD 9.99.75 on the same machines and installation in UEFI mode succeeded.

I have also tested installations under QEMU with either UEFI or legacy BIOS with identical results, so it is very easy to reproduce these issues if needed.

I have no clue as to why some OS boot on some machines and not on others while other OS boot on any machine. However, solutions exist and get implemented, and that's good.

20-100 I've just made a quick test with OpenBSD 6.8 and I confirm the installation ISO fails to boot in UEFI mode.

For some reason laptops seem to do UEFI better than desktops. Tried the USB install image and USB boot?

20-100 For the 'e' slice, you're right, I've got mixed up with NetBSD, sorry.

You have slices instead of wedges on NetBSD too? (I feel like I'm discussing cakes for some reason.)

rvp Tried the USB install image and USB boot?

When installing on bare metal, I always use USB install images.
I only use the ISO to create VMs.

rvp You have slices instead of wedges on NetBSD too?

Before hearing about NetBSD, I though wedges were just fried sliced potatoes...
That said, I just use sysinst to partition my disk, so I have whatever it creates for me.

The visible difference between the non-working version of sysinst in NetBSD 9.1 and the working one in 9.99.75+ is a new option surprisingly (and incorrectly) labelled "Delete everything, use different partitions (not GPT)" allowing to successfully and reliably install NetBSD in any context (bare metal, VM, UEFI, legacy BIOS).

[ The option label is incorrect because when selected, it correctly wipes the disk out (which earlier sysinst versions failed to do) and prompts you for the partition scheme to use, including GPT. ]

20-100 I just use sysinst to partition my disk, so I have whatever it creates for me.

A mount command or dkctl wd0 listwedges will tell you what you have. If your devices start with dk then you have the newer-style wedges. If you have devices ending in a, d, ... (like OpenBSD), then you have slices.

20-100 The visible difference between the non-working version of sysinst in NetBSD 9.1 and the working one in 9.99.75+

Lucky you. Even the latest 9.99.77 tells me bizarre things after looking at my partition layout (the wedges are not in order--they're layed out all over the place). So I'm still installing my NetBSD manually.

With an UEFI installation, I have wedges.

chrisfromgreece "There is a partition you can't touch partition C what is this for ?""
C is the designation for the entire disc.

I do not KNOW where the efi files are, but I assume they are in /dev/sd0a, which is where the boot loader is. I have never had to worry about it, because I set the BIOS to legacy mode which will boot both efi and non-efi systems.

I do not know why. I do not see how the boot loader would have any effect on a system after booting. If "can't start xorg" means the GUI does not load and the system returns to the TTY, create a file
/etc/sysctl.conf
and add machdep.allowaperture=1
Some hardware require that addition for the kernel to access drivers. Again, I do not understand how the boot loader could affect a running system, but I am not an expert. So it is worth a try.

I have never used uefi boot and so have never had a need to research the matter. All I can say is that many people do use uefi with OpenBSD.

chrisfromgreece I have ported some applications that are not available on openbsd do you know if i can somehow get involved with porting ?

Very much like NetBSD, there is OpenBSD-wip ports, https://github.com/jasperla/openbsd-wip

I don't know if it works the same way, but I'd suspect it does.

chrisfromgreece If you have the knowhow it is definitely possible. Many of the project's ports are created and maintained by individual users. If you are interested check the OpenBSD website.
[https://www.openbsd.org/faq/ports/testing.html]
[https://www.openbsd.org/faq/ports/index.html]
A few people who maintain ports are also active on the Daemon Forum.
[http://daemonforums.org/]