The way you would do this depends on how your database is implemented. It sounds like you have a Master Object, "Order", and a child object, "Order Line Item." I'm going to answer your question based on that assumption.

In your controller, you'll need to query the order line items. You can then reference the list of child objects directly in Visualforce using the API child relationship name.

For instance, if you wanted to query all contacts belonging to accounts, you would do the following query:

[SELECT Id, (SELECT Id, Name FROM Contacts) FROM Account]

Notice two things here: The nested SELECT statement, and the relationship name for the Contact related list, "Contacts."

Your query will probably look something like this:

[SELECT Id, (SELECT Id, Name FROM Order_Line_Items__r) FROM Order]

You can then iterate throught the line items of the order using {!order.Order_Line_Items__r} in conjunction with an apex:repeat or another iterative element.

(I've never tried iterating through with a repeat and then using a nested repeat to iterate over a sub-list, but I have no reason to think that won't work)

To find out what the relationship name is, go to the child object (Order Line Item in your case) and click on the Master Detail field for "Order". The Child Relationship Name is listed on that page under the Master-Detail Options section (make sure you append '__r' to it in your code.

If my assumptions about how your objects are set up is wrong, please clarify how you are set up so I can answer your question better.