I'm running on a Nexus 5X, "O" beta 3
Initially I found that I was locked out completely.
This happened randomly while the pebble is connected, and invariably if the battery on the Pebble was allowed to run down completely.
The latest update of "O" seems to have stabilised things, I have full access to the Pebble App and the Apps that I loaded. The only irritation is the loss of the timer which is an old problem, so I'm resigned to that.
Occasionally the watch drops the phone or vice versa. I go to the watch first to reconnect fromn there. If that doesn't work, I go to the app and reconnect from there. It's been stable now for about a month - fingers crossed!
If you do have problems, here is my work-around strategy ~ it might work for you too:
Uninstall the Pebble App
Reinstall. I would get through the bulk of the settings, including the Health settings before the App "disappeared" again.
It felt like a new installation that had gone wrong ~ (it had, but) it hadn't really
Left the two devices to ponder ~ went to have a cup of decent coffee
If it hadn't made the connection, go to the Pebble watch and select Settings, Bluetooth, your device
In my case, I found it had connected, and all my old settings had been restored, I could even find my old watchfaces.
All the data was gone though - Health, notes etc.
And timer no longer worked - permanently.
~ I was locked in the past. Couldn't change notification choices, update existing apps, add new ones
~ Timer gone
~ If the watch and the phone were seperarated for an hour or more, I had to reconnect from the watch