Orlando is a bit of an oddity in the respect to international arrivals due to the way luggage is (or at least, can be) handled.
Presuming you are not arriving from a Pre-clearance airport, then yes, you will need to re-clear security as well as immigration and customs in Orlando.
The flow in Orlando will depend on whether you have bags, and whether you are able to check your bags to your destination or not.
Upon arrival, you will go through immigration, collect your bags (if you have checked any), and then go through customs. If you have bags, and they have been checked through to Miami, then you can then re-drop your bags at the baggage drop, hop onto the train to the main terminal where you will re-clear security at the general TSA area, and head to your gate.
If your bags were not checked to your final destination then there's a few options. If you have at most one bag, then you can take the bag with you on the train, and re-check it with your new airline at the main terminal.
Alternatively - and there is where Orlando is different to most airports - you can drop your bag at the baggage re-check area and it will be transported to the baggage carousel in the main terminal for you. You then catch the train to the main terminal, go to the baggage claim area, and re-claim your bag before checking it in with your next airline. Doing this will take longer than taking your bag with you on the train, but it may be easier, and is mandatory if you have more than one (large) bag.
You can find a brochure that covers some of the details for this process on the MCO website, but keep in mind that this is really designed for arriving passengers, not connecting passengers.