GPIO pins of CC2650 are capable of driving the opto-coupler(4n35) directly?
The 4N35 is designed for a current of 10 mA.
It is possible to use a smaller current, but then there is no guarantee for the current transfer ratio, so you'd have to use the datasheet's typical values and apply a large safety factor.
For example, for IF = 5 mA, the datasheet claims that a collector current of about 6 mA might be possible, but to account for manufacturing variations and temperature effects, better make the output resistor large enough so that the collector current is no more than about 1 mA:



The CC2650 is capable of driving 4 mA on all GPIOs, or 8 mA on high-drive GPIOs. (The VOL/VOH specifications in the datasheet show how large the internal voltage drop can be at these currents.)
The "GPIO pullup current" specification describes the built-in pullup resistors.

If possible, replace the 4N35 with a better optocoupler: the PC817 (or any of its many "xx817" clones) is specified for 5 mA, and the H11L1 would work at 1.6 mA.