Multiply textbox with cell

1) Ditch the try-catch-block. It can detect errors only accurate to "somewhere in that block", you may need more accuracy. And just swallowing the exception in an empty catch block doesn't solve anything. It just hides the real problems and prevents you from fixing them.
2) int.TryParse(OdrQuantityTxtBx.Txt, out a) has a try-catch included. It returns a boolean you can test for successful parsing.
3) dataGridView1.Rows[i].Cells[1].Value is a string. You cannot just cast it to int. You need to incorporate 2) again.
4) c = dataGridView1.Columns[i].Cells[1].Value; overwrites your precious multiplication result from one line earlier with a cell value. I think you want it to be the other way round.
But that cell is the same you originally used for the multiplication. You may want to store the result in another cell instead. And .Columns[i] doesn't have a member named "Cells".
And I'm still assuming .Value to be of type string, so you can't assign it to an integer, nor a double.
So maybe have another column and use
dataGridView1.Rows[i].Cells[2].Value = c.ToString();

private void dataGridView1_CellEndEdit(object sender, DataGridViewCellEventArgs e)
{
    int a;
    int b;
    foreach (DataGridViewRow row in dataGridView1.Rows)
    {
        if (int.TryParse(OdrQuantityTxtBx.Text, out a) && 
            int.TryParse(row.Cells[1].Value.ToString(), out b))
        {
            // your code was overwriting the value that was in one of the 
            // cells you were using for the math. That seemed peculiar to 
            // me.
            row.Cells[2].Value = a * b;
        }
        else
        {
            // handle the error by either throwing an exception or displaying an 
            // error indicator in the cell that contains the result. 
        }
    }
}