There's a niggling issue with OPC, where after entering personal details, it scrolls to the cart contents, past the shipping/payment section, so users have to scroll back up.
For a quick fix, you can edit /skin/common_files/modules/One_Page_Checkout/ajax.checkout.js
AFTER...
Code:
ajax.widgets.checkout.obj.prototype.unblockSection = function (elem) {
$(elem).unblock();
INSERT...
Code:
$('html, body').animate({ scrollTop: $('#opc_shipping_payment').offset().top - 15 }, 300);