Hi there,
Hopefully i can complete the other answers just to let you know the difference. There are several conditions on how you can use "orang tua" in many sentences. But the regular use of "orang tua" terms are commonly for people who are over 50 years old. For your information it's different from "Adult" because it means "dewasa", but maybe you already know about it.
So back to the "orang tua" terms for "old person", you can use it like:
1. Orang tua itu sedang berjalan kaki di trotoar (that old person is walking at the side walk)
2. Orang tua itu terlihat sangat sehat (that old person looks very healthy)
But if you want to use it for "parent", you just have to put "saya/aku, anda/kamu or dia/ia " (i.e. my parents, your parents, atau his/her parents) after the "orang tua" for showing ownership.
So the key is whether you use the ownership words or not.