Hide "Cash on Delivery" on the order-pay page, and hide "PayPal Standard" on the main checkout page.

functions.php

<?php

/**
 * Hide "Cash on Delivery" on the order-pay page, and hide PayPal on the main checkout page.
 */
function wc_ninja_conditional_gateways_for_order_pay( $available_gateways ) {
	if ( is_wc_endpoint_url( 'order-pay' ) ) {
		unset( $available_gateways['cod'] );
	} else {
		unset( $available_gateways['paypal'] );
	}

	return $available_gateways;
}
add_filter( 'woocommerce_available_payment_gateways', 'wc_ninja_conditional_gateways_for_order_pay' );