Wrist heart rate monitor WHR inaccurate

Mine behaves the same as you described. The OHR works fine until I start an activity, then it starts showing higher HR numbers. So, for example, I am on the first screen of an activity that shows the timer and the OHR. Before starting the activity I would walk around and the OHR will behave normally. Once I start the timer, the same walking will show a much higher HR. If I stop and delete the activity the OHR widget will still show an abnormally high HR until I power cycle the watch. This does not happen with an external HRM. This is definitely a bug, and prevents me from using the OHR for my indoor workouts (such as Yoga, Cardio..).

Mine is pretty poor - it's generally reading about 15-20 BPM below my previous watch (910XT with a strap) and even seems to be going in reverse at times - I run harder and it reads lower.  Very disappointing.

Well my fr235 and Fenix 5 didn't behave like this. I'm not saying they were good with the OHR but they never behaved like the 945 jumping 50+ bpm right when you start the recording. I did multiple tests with the Fenix 5 OHR and comparing it to HRM TRI and to Wahoo TICKR X with different activities and the OHR were almost everytime off with the lows and highs and not very reactive  to fast changes in heart rate BUT surprisingly close with the average hr for the whole activity. The 945 is totally off as it might show 180+ bpm just when I'm standing still(I'm not that out of shape!) and the averages being 150-180 bpm when they should be between 100-130. 

No, your $200 Fitbit didn't track your heart rate accurately.  Nobody has manufactured a sports/fitness device with an oHRM that tracks accurately.  Due to fit, your Fitbit may have tracked better (even this is nearly impossible to know over a 24 hr period that may include activities) than your 945 does, but your Fitbit definitely wasn't accurate since the technology isn't there yet when it comes to an oHRM.  

How is it that just starting the timer changes significantly the HR readings? The same exact physical activity produces two significantly different HRs (from the 70-80 to the 110-120) just because I started the activity timer! The high readings remain in effect even in the HR widget until a power cycle!  That doesn’t make any sense. My Fenix3HR and Forerunner 935 did not behave like that, not even close. In a non activity scenario, the 945 OHR is well behaved even during a workout.

No, that doesn't make sense and your watch shouldn't be doing that.  If I were you, I'd call support as something may be defective with your watch.  I just tried what you described by starting the run app, walked around a bit and I'm not having this issue.  My HR reading rose 1-2 bpm from what it was while walking around before I started the activity.

I remember reading somewhere that the OHR gets fed more power during activity recording to improve the accuracy than it does in the all day state. Correct me if I'm wrong. Perhaps we do have faulty hardware which doesn't play well when it gets more power? I'm assuming everyone is still on the same firmware 2.40 and WHR 20.04.12?

I'm on the same FW and like I stated earlier, I'm not having this issue.  Is this something that happens every time?  I always use a chest strap for all my activities, but I've tested it 3 times now with the oHRM and my HR isn't jumping once an activity is started.

Thank you very much for performing the test. The plot thickens.... I decided to redo my test with the run activity. As in your case, everything was working properly. The OHR variability was bigger than I expected, but stayed within a reasonable range (I was actually just walking) and peaked at the low 80s. I then tried again my Cardio app, where in the first screen has just the timer and HR and what do you know, it behaved strangely again where the HR went all the way up to 112 without much of a physical effort. Since the same happened to me in the past with the Yoga activity, I don’t think it is related just to the Cardio app. I am going to delete and reload the Cardio app and see what happens.