I get all that. What I mean is in the Vanilla Oblivion when you use a skill let say a Heal spell, when you use it it gain some percentage or exp., or just by jumping you earn some exp for acrobatics. How do I do the same thing in Oblivion XP so I can use a spell that requirement 25 or 50?
I'm not quite sure you do. In vanilla oblivion, each time you use a skill (say, casting a fire spell for destruction), you have the potential to raise that skill by one point. In the background, oblivion is keeping track of the number of times you've cast a destruction spell. After you reach a certain threshold, you gain one skill point, and the threshold for the next skill up is increased by some exponential value (say from 3 to 8). Furthermore, each time you raise a skill, you are also "banking" points towards the governing attribute. So, continuing with the destruction example, if you raise your destruction skill by 10 points, you get a chance to raise your willpower attribute by 5 points when your character levels up. Furthermore, in order to "level up" in vanilla oblivion, you must raise your skills by 10 points in one or more of your primary skills.
The problem with this system is that if you want to level up certain attributes, you may be forced to level skills your character doesn't really use in order to get the most efficient boost next time your character levels up. For example, if you want to raise your strength attribute the maximum amount each level (which is 5), you must level a combination of blade, blunt, or hand to hand by 10 points each level. This is not very practical if your character is say, a mage who depends on long-range casting instead of trying to be Tamriel's next Ultimate Fighter.
OblivionXP does away with this system entirely (i.e. you no longer get skill increases by using the corresponding skill). What it does instead is award experience points to your character based on the actions you perform (killing, finding a new location, picking a lock, etc.). Once you have enough accumulated experience points for your current level, you can then "level up" and assign attribute and skill points yourself manually.
EDIT: As I mentioned, vanilla oblivion doesn't use a traditional experience points system at all. Instead it "counts" the number of times you've performed an action (cast a spell, jumped, hit a target with your sword, etc). If you would like a better insight in how the vanilla system works, open the console and enter "tdt," which will show you a list of oblivion's current counts for your actions, as well as what the next threshold is for a skill increase for each one. If you close the console, this list is stilled displayed (until you enter "tdt" again) and is updated in real time as you are playing. So, you can see what effects your actions have on each of the values.