Sorry to say Chris, but you got almost everything wrong.
1. you should limit the current through the LED with a series resistor. What
you did will destroy the diode immediately.
2. the phototransistor is connected wrongly. the emitter(pin4) goes to gnd,
the collector(pin5) to the gate.
3. The IRF9234 has 140m ohms channel resistance, with 10A this will make 14W
dissipation, wow. Take a IRF4905 instead.
Change the optocoupler to 4N36, which has 100% current transfer ratio
instead of 30%. With 10mA input current, the 4N36 will sink 10 mA, so to be
on the safe side, choose a R= 12V/3mA. A 3k9 will do
This circuit will work however a rf stabilizing resistor should be added in
series with the gate ( 300 ohms typically ) Every fet conducting currents
over a few amps should have such a resistor.