Equipment Enchantment
If you already play Lineage 2, you probably know about this, but for the olympiad this is a huge factor, because it influences directly in the dealt damage (weapon enchantment), and taken damage (clothes and jewellery enchantment). In most servers, you can safely enchant every equipment up to +3, and after that the enchanting chance is progressively reduced as the enchantment gets higher.
I can't tell you exactly how much each enchant increases your status or the enchantment chances, because administrators tend to change this to match the server rates and difficulty. The enchant limits also changes according to the server, but you can find these informations on the server's website, just like the availabe enchantment for the olympiad, because sometimes they make a special enchant limit only for the olympiad games, being usually +6.
Except by dual swords that gets their Special Ability when they reach +4 enchantment, and +6 clothing sets, enchanting your jewels will only increase your magical defense, enchanting your clothes will increase your physical defense and enchanting your weapons will increase your physical and magical attack.
+6 Clothing Sets Bonus
When you get all pieces of your clothing set +6 enchanted like in this image, you get a few more bonuses. If it's a clothing set that gets completed with less than 4 pieces, then you can enchant only the pieces that are part of the set to get these bonuses. Check the bonuses and values in the sheet below:
Normal Enchant Scrolls
Mostly the normal scrolls are used only untill +3, because after this, if the enchantment fails, your equipment will become a bunch of crystals, so it's a little risky to use them for higher enchant levels, even though they have a increased success percentage, something about 5% more than blessed scrolls in most servers, but feel free to try it if you think you are lucky enough.
You can get them through a few quests, normal drop from some monsters, drops from raid bosses, spoil, opening treasure chests with their respective keys or the skill Unlock from daggers, or with Noblesse Gate Passes, but this last option usually is not worth it.
Blessed Enchant Scrolls
The blessed scrolls are the ones you must focus on, because of their cost-benefit it's the best choice. If the enchantment fails, your equipment won't break, but the enchantment levels will regress to +0, or to the safe enchant +3, it depends on the server configuration.
They might be obtained from drops of raid bosses, exchanging Ancient Adena and Blank Scrolls in the Merchant of Mammon, through the Seven Signs event, or with Noblesse Gate Passes (this option usually is not worth it). Most servers have been giving more options to obtain the blessed scroll, so you should check if there is another option where you play.
Crystal Enchant Scrolls