Testing numbers using 'switch'
I haven't been a C guy for years, but a few observations:

You have int oddArray[oddIndex], evenArray[evenIndex]; declared, but the only time you use it is to add the entered number, and even that will fail because you have basically declared them as size[0], so oddArray[1] will throw an exception.

I think [even|odd]Index and [even|odd]Total can be combined into one variable.

I'm not sure what the standard is for C, but I really don't like the multiple declarations on one line.  There should be a single declaration per line.  Its less confusing that way.

You could also put some error handling around the input.  What happens if somebody enters "A"?  Your program would crash.  I also think your instruction should be a little more clear.  What does "please enter some numbers." really mean?

Minor thing, you forgot to indent the line after the if's at the bottom.

I would also move the reporting to a different function and reduce duplication.

Your code could turn out like this:

#include &lt;stdio.h&gt;

void DisplayResults(char numberType[], int total, int sum);

int main(void)    
{
    int oddCount= 0;
    int evenCount= 0;
    int evenSum = 0;
    int oddSum = 0;
    int userInput;

    printf("Please enter a number followed by &lt;enter&gt;.  Enter \"0\" to exit: \n");

    while (((scanf("%d", &amp;userInput)) == 1) &amp;&amp; (userInput != 0))
    {

        switch (userInput%2)
        {
            case 0:
                evenCount++;
                evenSum += userInput;
                break;

            default:
                oddCount++;
                oddSum += userInput;
                break;
        };
    }

    DisplayResults("Even", evenCount, evenSum);
    DisplayResults("Odd", oddCount, oddSum);
    return 0;
}

void DisplayResults(char numberType[], int count, int sum)
{
    if (total &gt; 0)
    {
        printf("%s total: %d, %s average: %d\n", numberType, count, numberType, sum/count);
    }
    else
    {
        printf("There are no %s numbers!\n", numberType);
    }
}


Please excuse me if I have the syntax wrong, I haven't used C in about 20 years.