Was disappointed with the results, look at how it's way off at the beginning an end:
Any tips to avoid erratic readings?

Hello,
I have the same kind of issue where the OHR just "unlock" and show completely a steady inadequate value (too high around 160bpm and very still completely not related to the effort). Even if I remove the watch it still shows such a value...
I wrote to Garmin support without an answer yet...
Do you think my OHR is defective ?
Could be defective, but try this first: Really prevent any outside light from getting in between your wrist and the watch. It takes a while for the watch to recover from wrong oHR readings because oHR is based on some sort of rolling average.
Try strapping it on unnaturally tight, almost but not quite painful.
I find it also helps to let the watch "settle" into your skin by wearing it for hours before your run.
If you waited for the solid heart symbol before starting your workout and it still shows erratic readings, I would return it.
PS You can compare oHR to your chest strap data by adding this little Data Field in an additional screen:
https://apps.garmin.com/en-US/apps/88ce4547-7d84-4289-b2c7-3e15ca00185f
Turn the chest strap off on your watch, the Data Field will then pair to it automatically. The data field does not need to be actively on screen to do its thing, works nicely in the background :)
Yesterday I made another running test yesterday wearing my watch really really tight and making sure my sleeves wouldn't bring any moist on my wrist. I struggled to have a proper OHR "lock", the heart icon stopped blinking but restarted 3 or 4 times. Finally it was still for enough time allowing me to start my activity but it failed again...
I made a return request to Wriggle and wait for an answer... I quite pissed having spent 550€ for a high end defective device...
