Then what, if any, aarmor enchantments can be used to buff damage? Obviously a weapon can have on-hit damage, but armor enchants seem to favor defensive options only, unless I am missing something.

Shield can reduce Damage, spells like Shield Wall can give elemental damage from this protection. I don't think there is a Reflect spell effect in this game, but that could give you what you want. Look in Alteration spells

http://www.uesp.net/wiki/Oblivion:Spells

But, you don't really need to over max out damage or protection in this game. Nothing is that strong, unless you have mods that add things.

Doing an archer character right now and it just feels like my arrows peck away little teensy bits. My destruction spells are stronger but honestly if not for my chameleon abuse I'd get my ass handed to me. XD

If you are playing vanilla, enemies can have a LOT of health at high levels. If you have maxed out Marksman and Agility and are still having trouble, you can enchant the bow or use poison. You are correct that oosting skills or attributes over 100 won't add to damage in the vanilla game. You could try to get your armorer skill to 75 and then you can improve your weapon to 125% or finding a better quality bow, if you don't want to enchant it or use poisons. Other strategies include using magic in combination with your bow.

I guess my real question was if "increase bow damage by X" enchants like Skyrim's were a thing. Guess not. XD Thanks for the tips though!

Things to consider: What kind of bow are you using? What kind of arrows? What's your Marksman level? Are you using poison? Are you making your first shot from a crouch (Sneak) to get the Sneak Attack critical? Is your bow in good repair? Is it enchanted, and if so, is it charged? (I realize some of these questions may be trivial, but I don't know your experience level with the game.)

My current character, Phoebe, is an Archer/Nightblade. Her Marksman is around 80. She uses an Amber bow enchanted (Grand Soul) with Soul Trap, Weakness to Magic and Health Damage, usually with Glass (or better) arrows, and a poison. The bow and arrow alone will do about 30 damage, and the enchantment amounts to another 30 points. Add to this her poison, which at her current Alchemy level does 7 points of damage per second, for 27 seconds, and you've got a total damage output of around 250 points on one arrow (not counting the sneak-attack critical.) I would hardly call it "pecking away."

I'm not at that stage yet. Using a glitch-obtained Bound Bow with Steel Arrows, level 15 right now about 55 marksmen and 70 agility (I'm a Breton so initial agility stat was low) give or take. Haven't been diligently poisoning or maintaining charges, maybe that's my issue. Been mostly using base damage.

Poison would definitely help. As would making sure that first arrow is a sneak attack.

There are no "increase weapon damage" enchantments in vanilla Oblivion. However, the fortify Marksman and fortify Agility enchants (and spells and for Agility there are also potions) will increase bow damage up to the point that your Marksman skill/Agility reach 100. If you have 55 Marksman and 70 Agility, you can increase your bow damage considerably by fortifying Marksman Skill and Agility. You get more bang for your buck with a Marksman skill fortification than an Agility fortification. So I'd start there and then once you fortify that to 100, work on Agility.

Of course, the strategies glargg suggested might be more effective, but fortification of skills/attributes will increase damage up to the point those hit 100. Further fortifications beyone 100 will not affect damage in vanilla Oblivion.