Wow Lisa, that's a tough one! If it weren't for the -OPKs I'd swear you Oed back on CD10. However, with the +OPKs & CM, I'm going to guess that it was more like CD15/16. What's your CM been like since CD15/16? If its no longer watery or EW, I'd say you probably Oed then.
Did you try using the temperature adjuster on FF for the last 2 temps? It doesn't seem like 15-30 mins would make a huge difference, but you never know. If I use the temp adjuster I just make a note of my actual temp & time in the notes section.
We'll have to have a POAS party around Halloween!! Good Luck!