Zu CKOPT steht doch alles im Datenblatt:
Zitat Zitat von ATMega8 Datasheet
The CKOPT Fuse selects between two different
Oscillator amplifier modes. When CKOPT is programmed, the Oscillator output
will oscillate a full rail-to-rail swing on the output. This mode is suitable when operating
in a very noisy environment or when the output from XTAL2 drives a second clock
buffer. This mode has a wide frequency range. When CKOPT is unprogrammed, the
Oscillator has a smaller output swing. This reduces power consumption considerably.
This mode has a limited frequency range and it cannot be used to drive other clock
buffers.
For resonators, the maximum frequency is 8 MHz with CKOPT unprogrammed and
16 MHz with CKOPT programmed.
Bei einem 16MHz-Quarz musst Du CKOPT also auf jeden Fall setzten.

Gruß,
askazo