Well, I've just started a new game that I hope is going to take me up to November and I have NEVER used Marksmanship before so I figured I could give it a go. I've only got it at a minor skill and at the moment its about level 10, but I can't find anything that I can kill in ONE shot apart from rats! I've tried going after deer with steel arrows (bear in mind I've only been playing this game for about two hours and am still level 1 and running OOO) hit them once, their health plummets but they run off! I need one more hit to kill them but they've gone and I can't keep up!
I've even tried...cheating! Toggling God Mode then just firing hundreds of arrows into the bushes, but I still never kill anything

If steel arrows won't do it, where can I get another type of arrow to help me go hunting, I need the venison for my alchemy

What level are you? as you level gets higher you will find better http://www.uesp.net/wiki/Oblivion:Arrow

use poison

Shamefully I've only just reached level 2....But that's because I'm using OOO with the thing to slow down levelling, plus with no fast travel and most of my Major Skills being combat based, enemies are few and far inbetween

With Marksman as a major, I only started one-hit killing humans and other comparable enemies at about level 20ish. But only when sneaking. At level 30 (my character's cap) it's still difficult to one-hit kill while not sneaking, as Gimme said, use poison.

And I second. Preferably custom-made ones

Yeah..... one-shotting.....

The only time I see unassisted one-hit kills (that is, with just an ordinary bow and ordinary arrows) is with a character with notably high starting marksman skill (like a Bosmer with marksman as a major), with the best possible equipment, with a successful sneak hit with sneak of at least 25 (so that it's a 3x bonus), against the weakest enemies (bandit archers and wizards, imps, mudcrabs, rats, sometimes deer), in the first level or two. That's it. If the character's skill (marksman or sneak) isn't high enough or his equipment isn't good enough he'll never get unassisted one-hit kills at all, but even if he does, it'll only be for the first level or two. Then the rest of the world scales up enough that base marksman damage just isn't enough to take out much of anything in a single hit. And that's when poisons become important. There are some powerful enchantments that help too, but poisons are key.

The character I use now is a wood elf archer, I started with agility as a favoured attribute to do more damage with bows right off the bat, also the thief birthsign that gives you +agility too and marksman as a major so I was 1 shotting enemies early on in sneak mode( major too) I got agility to 100 pretty fast ( I leveled efficiently)
Just an idea if u fancy starting over as u r not too far in , I restarted my first archer 25 hours in and happy I did .

At the default difficulty setting my archer is 1 shot sneak attack killing bandits still at level 20ish with 90ish marksman and sneak skill. At 100 sneak I gain the ability to ignore armor with sneak attacks, i think, so at that point I will be able to 1 shot marauders and things too. I only use poison on spell casters because they run around while healing themselves so often that it becomes very annoying.

It is only a few of the bandits. Usually its 2-3 hits.

At level 30, with Marksmanship at 100, I one-hit kill all spellcasters, save none. I use poisons on the heavy armor guys.

Just started playing Oblivion myself, also have Marksmanship as one of my major skills, finding the aiming a bit stiff at the moment but I'm sure I'll get used to it. Not sure if there is an option to highten the sensitivity (PS3)..

If you really wanted to one shot enemies you could always lower your difficulty to the lowest setting.

Since you're using OOO head over to the Inn of Ill Omen (on the road to Bravil from the IC) and try catching the Legion Forester target practicing with his bow (I think he either does that around 6pm). I thought I saw him using elven arrows when I was at low levels.