Our Orange Tree is Losing It's Leaves
The first photo shows that your orange tree is infested with scale insects. You can see the brown dots along the leaf margin. This is a serious problem of citrus plants and difficult to control. Usually, when grown outdoors this insect is kept in check by natural enemies like ladybugs that feed on the scale.
Your best option is spraying the plant with a horticultural oil that is labeled for houseplants. Wait until the weather cools off a bit and move the plant into the shade before doing so. You need to target the undersides of the leaves. There are also systemic insecticides labeled for scale on houseplants but most likely they are not to be used on fruit. You need to get this under control before you move it back inside for the winter. There is still time before you need to do that allowing you to treat the plant about 2-3 times before moving it.