Vaughn Mancha is an Old Earth Creationist who frequently posts on Facebook.
Here is a snap of one of his posts, in which he defines the key characteristics of Old Earth Creationism.
Note two major points: the purpose is to harmonize biblical narratives with scientific consensus, and that the “days” in Genesis are actually vast periods of time.
We’ll start with the second point. The Hebrew word “yom” can mean a regular day, and depending on the context, can also mean something more abstract, like an era. The Answers in Genesis organization points out that since Genesis uses a number with “yom”, the meaning has to be a regular day.
The clincher for me is Exodus 20:8-11. In that passage, “yom” has to mean that the days of creation week were regular days.
Your beliefs about the age of the earth should be based on the text of scripture.