Another possibility: what is the supply voltage tolerance for your oscillator?. Suppose it is happy with 3.0V. Then, even if its high output level is 3.0V, this won't bother your 2.6V I/O, no internal junction will be significantly forward biased.
Otherwise I would vote for 1: resistor divider, with a C across the series R. Use the same criterion as for 10x scope probes compensation: the added C will compensate the input C of the I/O. Choose as low Rs as possible consistent with output driving capability of your oscillator, especially in case the I/O has an important DC component (e.g. if it's a TTL gate).