You can exceed the cap by bonuses by 7.5% of the upper limit which is $4.8 million this season.
The Oilers have $9.1 million in performance bonuses thanks to RNH, Yakupov, Schultz, Belov and Petry's ELC. $4.3 million of that can't be used in the bonus cushion and still will count towards the $64.3 million limit.
Source:
http://capgeek.com/faq/what-s-a-bonus-overage
http://capgeek.com/oilers/
Capgeek's cap calculator doesn't take into account if a team is exceeding the bonus cushion like the Oilers are.