Omni's signature skill, this skill makes early game easy and makes you stay in lane forever. I find this skill works out very well with arcane ring. Purification is also a nuke bou you're enemy has to be about 200range beside you to tkae damage, but that is what Degen aura is for.
Purification should be maxed as by the time you are level 7. It is great spell since you can both heal and deal damage at same time.
Repel can make an unit spell immune for 20 seconcds at lvl4. The dowside of this skill is that once a unit is spell immune, you can't cast Purification.
It is important to get Repel as a first skill. Reason for that is because, usually, it is more useful on first two levels. You can can cast it on your allies or yourself in order to prevent magical attack. Next time I skill Repel is at level 12. I do this because I prefer to cast heal on my team mate rather than let them be immune to magic 2/4/6 seconds. Only time when you should skill Repel before Degen Aura is when your opponents have a lot of skills that deal magic damage.
AOE slower. I ususally get this in early game to draw first blood, but getting it in mid game is also okay. The slow stacks with other slows like Skadi.
After Purification I max Degen Aura. It is great skill to kill your opponents when you and your carry are ganking them. Since they cannot run away from you casting Purification on yourself or your carry is much easier.
This skill is just like a healing poison nova, except them your also immune to physical damage when you casted this. There are tons of ways to use this skill but it will mainly be used to save allies or yourself.
Guardian Angel is one of the best skills in game. Theoretically, you can join 5v5 fight and win it while no player in your team loses more than 50 hit points. You should max this skill as soon as you can.