I believe on the Toy Story 3 BD, the 7.1 mix is entirely new just for the home video release, while the 5.1 is the theatrical mix. So there could be more differences between the two than how the surround channels are handled.
Is your receiver is set to down-mix 7.1 to 5.1 when receiving the signal over HDMI? If that's the case, it sounds to me like there's a bug in the Oppo's firmware. Be it in the DTS decoding (kind of likely, it's had bugs in the past with both DTS and DD), or in the digital summing of channels that is causing the clipping.
I'd contact Oppo and report your findings. Check for firmware updates, and make sure it is repeatable first.
