Increases the chances of a Pokémon producing an egg.
Not good enough, huh? :P
God, dis was difficult. This is a valid answer so far, and I cannot find anything anywhere else saying otherwise.
The chances are changed every 256 steps.
After looking into the code with some tools of mine(calculation tools, editing tools, viewing tools, coding tools, etc.) (legality questionable?), I found some stuff on the Oval Charm. So, I ran calcs, and got a 10% increase the first time. After retrying it, it went to 11.25% increase. Tried it again. 5.75% increase, and other various numbers.
Yeah, I would post the code, but it is over 250 lines, so nah :P. I hope this is what you want. If Bulbapedia or someone credible says otherwise, I will hide. But I am sure this is correct. 100% positive.
Also, if you hide this after I post it, I will be pissed :P. Broke my first W2 cartridge trying to answer this.