We report on multi-color optical photometry in the 2015 outbursts of V404 Cygni, an X-ray transient. This system showed optical oscillations on timescales of 100 sec to 2.5 hours in these outbursts, and they were correlated with the simultaneous X-ray variations. On the basis of the $\sim$1-min optical delays against X-rays and multi-wavelength SED analyses, the optical variability in this system was likely caused by X-ray reprocessing at the outer accretion disk. We also find that some repetitive optical oscillations can occur at mass accretion rates $>$10 times lower than previously thought. Although the mechanism of this kind of variations is still unclear, we suggest that the lack of sustained mass accretion, which would be induced in long-period systems, may be a key condition.