Opto interface with LPC controller
Assuming you're using a 5V-tolerant output in 3-state mode, and that the 5V is only present when 3.3V is, then the voltage should be okay. 

Current will be about 7mA, which is almost twice the current at which the output voltage is guaranteed to be 400mV. Chances are very good that it will work okay, but a very conservative approach would be to buffer the output with something like a transistor. 

Given the implied supply sequencing requirement, it would simplify your life and eliminate a worry if you just put the transistor in there (to drive the opto). An MMBT4401 with a few K base resistor would work fine. The 1K resistor is not necessary- but if you wanted to protect the MOSFET gate (against nasties on the 12V bus) by adding a zener it would be useful. 

Your margin on CTR looks appropriate. 

Here's a schematic showing Q1/R1 to buffer the output and an optional 2-BJT MOSFET gate driver. Note that the transistor inverts the logic (GPIO high = ON) compared to your original circuit. 







simulate this circuit &ndash; Schematic created using CircuitLab

Outside the scope of this question, but you may need to add a driver circuit on the MOSFET side of the opto to make it switch faster. A couple BJTs would do it.