After enabling One Page Checkout plugin, you have to manage settings in NOP admin.
path: /Admin/OnePageCheckout/Configure
Select tab: General
Default country id - default billing address country id;
Default state/province id - default billing address state/province id;
NB! Configuring Default country id and Default state/province id enables admin to avoid nopCommerce standard logic (when state/province is not mandatory field and customer does not fill this field then is automatically added default values - Bangladesh (country id 19) to "Country" field and province Manikganj (মানিকগঞ্জ) (state/province id 76) to "State/province" field). When admin adds to Default state/province id value "0" then this field is not filled automatically with those default values and they stay empty.
Disable billing info fields - when is checked, then are billing info fields read-only;
NB! When customer choose courier shipping method, then shipping info fields are editable.
Show only business customer option - when is checked, then is in checkout enabled only business customer option;
Use custom Business Customer Form - when is checked, then on checkout form for business customer is shown additional field for registry code and address2 field is hided;
Extra rule of shipping methods available - when is checked, then if cart has at least one unshippable item, then all items in shopping cart become unshippable.
Calculate total - when is checked, then is order total always calculated, even if shipping amount is missing.