Blood oxygen accuracy

I hope 84% is not accurate or you would need to seek medical advice immediately.

It is supposed to be +/- 2%. Tattoos and loose/too tight fitting can affect all wrist readings

Apple say the Watch is for general health indication and should not be considered a medical device.

If I don't say this someone else will...

Contact Apple Support.

Click here > Contact - Official Apple Support

or phone numbers 

Click here > Contact Apple for support and service - Apple Support

For under £30 you can get a finger mounted oxymeter which will be accurate to 1%

Amazon has Pulse Oximeters for under $12, if you want a check.

As with any measurement, you can't rely on just one try. Lots of things can happen in 15 seconds. Have you repeated the oximetry measurement twice in a row? After readjusting the watch and band? Does it change?

If I'm careful, my watch has always been close to the "gripping-finger" value.

I can slide my pinkie inside the band but not easily my index finger

Same here