Using N Channel Mosfet (non logic level) for 24v 15A dc motor pwm control

can i use optoisolator 4n35 to step up the gate drive voltage to 10volts using external power source?

Use the open collector output of the opto to hold the Gate low to turn it off; use pullup resistor from Gate to 24V to turn it on.
Make sure the MOSFET can handle that big a swing - otherwise use voltage divider to limit how high the gate is pulled up.