I'm sure there are good guides already on how to actually install the oil pressure sensor. I had to move the alternator out of the way to get the factory pressure switch out, but with the right deep socket you could probably get to it without removing anything else.

The new pressure sensor has two outputs. One is a switched output that works like the factory switch did. It connects to the wire that was connected to the factory swich. The other is the gauge output. It connects to one of the analog inputs on the openOBC.

With the sensor installed, it's just a matter of hooking it up, sprinkling some code, and hoping for the best.

tasks/ObcOilPressure.h

#ifndef OBCOILPRESSURE_H
#define OBCOILPRESSURE_H

#include <ObcUITask.h>

namespace ObcOilPressureState {
    enum state {DisplayCurrent, DisplayLimit, SetLimit};
}

//starts at 0mV; increments by 10mV
const uint16_t pressureLookupTable[] = {
    0,
    0,
    0,
    0,
    11,
    25,
    39,
    53,
    67,
    81,
    95,
    110,
    124,
    139,
    154,
    169,
    184,
    199,
    215,
    230,
    256,
    261,
    278,
    294,
    310,
    327,
    343,
    360,
    377,
    394,
    411,
    428,
    446,
    464,
    482,
    500,
    519,
    537,
    556,
    575,
    594,
    613,
    633,
    653,
    673,
    693,
    713,
    734,
    755,
    776,
    797
};

class ObcOilPressure : public ObcUITask
{

public:
    ObcOilPressure(OpenOBC& obc);
    ~ObcOilPressure();
    
    virtual void runTask();
    virtual void buttonHandler(ObcUITaskFocus::type focus, uint32_t buttonMask);
    
    virtual void wake();
    virtual void sleep();
    
private:
    ObcOilPressureState::state state;
    uint32_t pressureLimit;
    uint32_t pressureLimitSet;

    float getPsiFromVoltage(float voltage);
};

#endif // OBCOILPRESSURE_H

tasks/ObcOilPressure.cpp

#include "ObcOilPressure.h"
#include <ObcUI.h>
#include <cmath>
#include <cstdlib>

using namespace ObcOilPressureState;

ObcOilPressure::ObcOilPressure(OpenOBC& obc) : ObcUITask(obc)
{
    setDisplay("ObcTemp");
    
    std::string configState = obc.config->getValueByNameWithDefault("ObcOilPressureState", "DisplayCurrent");
    if(configState == "DisplayCurrent")
        state = DisplayCurrent;
    
    pressureLimit = strtoul(obc.config->getValueByNameWithDefault("ObcOilPressureLimit", "8").c_str(), NULL, 0);
}

void ObcOilPressure::sleep()
{
    obc.config->setValueByName("ObcOilPressureLimit", "%i", pressureLimit);
    if(state == DisplayCurrent)
        obc.config->setValueByName("ObcOilPressureState", "DisplayCurrent");
}

void ObcOilPressure::runTask()
{
    float currentPsi = getPsiFromVoltage(obc.analogIn1->read());
    
    
    if(state == DisplayCurrent)
    {
        setDisplay("oil: %02.0f psi", currentPsi);
    }
    else if(state == SetLimit)
    {
        setDisplay("set limit % 3i psi", pressureLimitSet);
    }
    
}

void ObcOilPressure::buttonHandler(ObcUITaskFocus::type focus, uint32_t buttonMask)
{
    if(focus == ObcUITaskFocus::background)
    {
        if(buttonMask == BUTTON_TEMP_MASK)
            obc.ui->setActiveTask(this);
        return;
    }
    
    if(buttonMask == BUTTON_TEMP_MASK)
    {
        if(state == DisplayCurrent)
            state = DisplayLimit;
        else if(state == DisplayLimit)
            state = DisplayCurrent;
    }
    
    else if(buttonMask == BUTTON_SET_MASK)
    {
        if(state == DisplayLimit)
        {
            pressureLimitSet = pressureLimit;
            state = SetLimit;
        }
        else if(state == SetLimit)
        {
            pressureLimit = pressureLimitSet;
            state = DisplayLimit;
        }
    }

}

float ObcOilPressure::getPsiFromVoltage(float voltage)
{
    if(voltage < 0.04)
        return -1;
    if(voltage > 0.5)
        return 999;
    
    float currentPsi = (float)pressureLookupTable[(uint)(voltage * 100)] / 10;
    return currentPsi;
}

Well, it works! Now I just need to do a more permanent wiring job and clean up the code.