Optimal way(performance wise) to store array like data in MySQL
If you just want somewhere to dump JSON then you're wasting your time with MySQL. Use a simple docustore. You will lose the ability to query the dataset in any meaningful way, but if storage and retrieval is your only requirement, there's no need for a relational database. 

If you have more complex requirements, then you will be far better off using a relational database. e.g. You need to know how many items of type X are needed to fulfill all open orders; You need to know how many open orders currently exist for a specific customer, and what the total value of those orders is, etc etc. Using a docustore and JSON will severely limit your ability to answer questions about your dataset.

Finally, you're describing a many-to-many relationship between orders and items, right? One order can consist of multiple items, and one item can be a part of multiple orders, yes? In this case you need three tables - orders, items and an intermediate table to join them: orderitems. This design is what we call "normalised" - it will allow you easily query the data while minimising the amount of data stored.