Actually you're only half right
Lets do some quick maths here
In all of the following cases, I will be using the same base damage for Oblivion Wing - this DOES NOT factor in anything else, and I'm inventing a number for the damage Oblivion Wing will cause and going from there. These results will still apply as an overall thing.
Also before I move on;
>Second of all, how much Flying Gem will restore depends on how much damage the move will do (e.g: it will do alot more damage to a Lilligant than it will do to an Aggron).
What Terlor said here is true - however the amount of restoration Big Root + Oblivion Wing provides will also fluctuate based on the Opponent's stats and resistances as the actual amount of health that is gained back (not percentage-wise) will fluctuate depending on the damage you deal.
Lets say for arguments sake, that Yveltal deals 100 damage (HP) to the opponent with Oblivion Wing.
With Big Root + Oblivion Wing, Yveltal gains back 97.5 Health
Now lets go to Flying Gem + Oblivion Wing
If we assume Oblivion Wing still deals 100 damage- however Flying Gem boosts the power of Oblivion wing by 1.3x
This means ROUGHLY SPEAKING Oblivion Wing will now deal 130 damage.
75% of that is 97.5 HP - it's a tie
Now lets go to Life Orb.
Life Orb increases the power of Oblivion Wing by 1.3x - this means Oblivion Wing will roughly deal 130 Damage instead of the normal 100 now.
75% of 130 is 97.5% - However we have not factored in the 10% Health Decrease from Life Orb - and right now, we are unable to do this since I've been calculating Health Gained from Damage Delt rather then paying attention to the Yveltal's total health.
So this option is the most inferior.
So to sum up - Tie between Big Root and Flying Gem. however do note Flying Gem is one time, and Big Root is not.
Also please note another factor;
Choice Specs Oblivion Wing will out-damage and out-heal all of the previous options by 15 HP.