Confusing apps!

Monica • I`m a 30 year old military wife and mother to our 9 year old son, Rylan and daughter, Maci who is 4.
This is why I take OPK's. Different apps says I'll O on different weeks. One says this week, one say next week, as glow sad the week after next. Confusing! But I'm interested in when I really O and which app is right. This is my first month using Glow.