When comparing the stats as the ability resolves, it's possible that the stat that's greater changes from power to toughness or vice versa. If this happens, the ability will still resolve and you'll put an oil counter on Evolving Adaptive. For example, if you control an Evolving Adaptive with two oil counters and a 1/3 creature enters the battlefield under your control, its toughness is greater so the ability will trigger. In response, the 1/3 creature gets +2/-2. When the ability tries to resolve, its power is greater. You'll put an oil counter on Evolving Adaptive.