Confusing apps!

Monica • I`m a 25 year old military wife and mother to our 5 year old son, Rylan and our sweet Maci born 7-17-15.
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.