OK. I have used OA in anger now so I have a better idea of what is going on.
Firstly, route planning. This a marked improvement IMHO. Using the website I plotted a route using roads. public footpaths (green dashed line), tracks and non PROW paths (black dashed lines). It was enough to set waypoints at the major turns, the softwaere automatically laid a route along the line of the path or road. This is akin to the snap-to feature in the OS app except that that only works inside national parks.
I saved the route as a plan then decided to go back and edit it. There is an edit option in the plan screen but that only lets you edit the title and description, however a double tap on the map opened the route edit option and I was able to move some waypoints and re-save.
Transferring my route to the phone was easy. Go to my page, scroll to the bottom and tap synchronise and the new plan willl appear.
Now to the in the field bit. I set off with the sound off so I was just using the screen display. The route was shown on an OS map. My position was shown as a grey dot but there was no vector arrow as in ViewRanger. There is however a nifty pull down window at the top of the screen giving navigation instructions. The pull down window is scrollable to give more navigation information and a route profile showing current position. If you want to see more map, the navigation window can be parked at the top of the screen.
Once I was happy I parked the phone in my pocket for a while. After a time the screen dimmed and then the phone went to sleep as it normally does. I walked for a km or so then reactivated my phone, it needs a PIN or fingerprint. The app had continued to record a trace and showed current position. All in all pefectly usable and a possible improvement on VR.
Then I tried to get lost. As you leave the intended route a red caution triangle appears on the navigation window after about 30 to 45m. At the same time a red line appears on the map showing you the shortest distance back to the route. Returning to the route kills the caution. If you do not have the app open, the caution is repeated in your phones information display.
Now to the voice bit. To get voice navigation the speaker symbol at the top of the screen has to be selected. This lets you pick one of your phones text to speech apps. If you go off route, you get a "You are about to leave your route" warning at around 30m and "You have left your route" followed by information on where the route is e.g. behind you, 50m S etc. As far as I can tell, the warning does not keep repeating but I was not off route for more than a few minutes. There is also a message telling you when you are back on your route. I did not get any spurious alarms even though the error seems to be set at around 30m. That is probably a quality of the phone gps issue. The voice navigation works even when the phone is asleep.
When in use. the voice navigation instructions are just like a car satnav with the usual "tiurn right in x metres" sort of thing and it seems the voice navigation package is all or nothing. No chance of just having the off course option. Not sure how I feel about that - I would have to try it in wilder country to see how often it talks. My impression it is just pipes up at junctions so it might be acceptable (I used to drive a route where at one point my satnav said "Follow this road for 520 km" then shut up for 5hrs!)
Hope this helps Jimbob