Approaching big O!!

Lisa • Married 5 years to a wonderful man. Son born 26 Jan 15. Expecting another boy in Nov 16 X
Ladies - please help.
I am in my fertile window according to glow albeit it is my first month using the app.
​I'm on day 3 of my fertile window (according to glow), I am using opk twice daily at same time (just the cheap strips) and getting a negative. My BBM has dropped the past two mornings but I have no signs off O (sore breasts, cramps etc). I'm finding CM testing hard due to BD and pre-seed over the past few days.
​What is the normal pattern for people when O is approaching?  Is there any chance the OPK will return a false negative when testing twice daily?
​I'm CD 15, normally cycles are 30-32 data long. 
​Don't want to miss it :-)  
​Thank you