PHP Use Declared array Variable inside already Declared Array

This is more a generic PHP question than anything to do with WordPress, but I’d suggest setting the value in the constructor:

class test {
    public $basicCols;

    public $optList = array( 'one' => 'One', 'two' => 'Two' );

    function __construct() {
        $this->basicCols = array(
            array( 'title' => 'KEY', 'field' => 'slug', 'options' => $this->optList ),
            array( 'title' => 'KEY', 'field' => 'slug' )
        );
    }
}