Between the complexity of Oblivion's leveling model and the variety of gameplay possibilities, it is very difficult to give advice. However, in this section I will try to give you a few guidelines and suggestions from my own personal experience and observations. Perhaps most important is this:
There is no wrong way to create or level your character!
Don't get bogged down in the numbers unless that's what appeals to you. Oblivion is a game that can be conquered many, many different ways. Remember to focus on having fun first!
Fun aside, the following sections should help give you a framework from which you can make an informed decision for your character's long-term potency. Basically, the process goes like this:
If you get stuck, poke around the Internet looking for Oblivion character templates. For example, there is a fairly detailed FAQ with several templates in GameFAQ's Character Build FAQ (PC) document. It is doubtful that any template will match exactly what you have in mind, but if you input the template info into OCP, you can see the exact impact of your tweaks.
Finally, as an abolsute last resort, don't forget that you can adjust the game's difficulty or (on the PC version) use the console to fix any mistakes you've made while leveling. If you are using OCP to plan your character, you probably don't want to resort to these tactics, but it's probably better to tweak something rather than scrap all the work you put into your level 35+ character.
If you are going to build a potent character, you have to understand what constitutes your potency. You can learn some of this information from just playing the game, but to really understand things you need to dig into the formulas and algorithms used. Here are a few references (in the rough order you should read them) that should help:
Note: If you are totally new to Oblivion, there is no way you will be able to understand all of Oblivion's mechanics. But that's ok -- just learn what you can. Remember that just by using OCP you are helping to ensure your character will be adequately powerful by game's end.
With so many different ways to solve problems in Oblivion, if you try to do them all, you'll end up a Jack of all Traits and Master of None. It's best to pick a few ways to solve Oblivion's problems and stick with them. Which solutions you opt for is your Gameplay Profile.
For each of the following obstacles, you should select one primary solution. The goal is to align your solutions among a select set of skills and attributes. You can then focus your efforts on those and largely ignore the rest. Don't forget that choosing to avoid the problem (e.g. not having any way to avoid fights because you want to kill everything!) or waiting until you are a higher level to address a problem (e.g. paying NPCs to recharge items later in the game because you can easily afford it) is perfectly viable.
Note: Scrolls are not considered a viable solution here because they cannot be reliably found in loot or bought from vendors. Although, scrolls you do find make great supplemental solutions, especially when you consider using them does not generate any skill XP.
There are a few other minor obstacles in Oblivion (e.g. breathing under water), but they don't really require specific character planning. You can usually find a solution without impacting your character's creation and leveling plan.
It is very easy to get overwhelmed by the multitude of character creation options, so this section will outline a few common do's and dont's. However, keep one overriding key point in mind:
These are generalizations, not rules. So long as you plan for the issue, you can find a way to safely violate these guidelines.
The following are key points for creating your character and forming your leveling plan:
To help illustrate the methods outlined in the Advice section, I'll run through the process with my example toon Nullis.
My general idea for Nullis was a stealthy, ranged assassin. I wouldn't rely heavily on magic or melee, but if I could find you at range, you'd probably be dead before you reached me. Plugging this general playstyle into the profile yielded the following:
With my profile in mind, I decided to use One Major Skill per Attribute as my custom class paradigm. This was appealing to me because I wouldn't level too fast or too slow and I could ensure that I'd have a major skill to level for every attribute; even when I'm leveling the least used attributes in the higher levels.
For my major skills, I chose a mix of skills I would use (on the attributes I wanted to level first), skills that I may use in later levels but were easy to level up, and skills I would never use except for leveling up. Here is the summary:
For my attribute Leveling Order, I didn't want my first few levels to go as fast as the +5/+5/+1 model would provide, but I didn't want to completely ignore Luck for a full +5/+5/+5 model. My solution was to do +5/+5/+5 for the first three attributes and by making Luck my fourth attribute, the remaining levels would be +5/+5/+1. This results in the following Leveling Order:
Note: In retrospect, I would probably have tried for a full +5/+5/+1 model by swapping Speed and Luck. I'd need to make sure I had enough Spare Skill Points in Speed because I'd be using those skills constantly at the lower levels, but the swap would get me to max level one level earlier (43 instead of 44).
My choice for race was made simple by wanting to use the Khajiit's racial Night-Eye ability for seeing in the dark. My birthsign choice was similarly simple: The Thief gives me a bonus to 3 of my top 4 attributes. So putting it all together, I arrived at this:
If you load Nullis' data and switch to New Character, you'll see these exact values which yield the following Analysis:
The max level is fairly low (many character configurations max at high 40's/low 50's), but since anything over 40 is largely meaningless, this keeps the excess to a minimum. All attributes get maxed at 100, so this character will reach the maximum possible potency. Since Endurance gets max'ed fairly early, Health is solid. And with over 3 Spare Skill Points per Level for every attribute, I don't have to sweat extraneous skillups. Overall, a solid design that should lead to a fun gameplay experience with more than enough character potency.