Did you try another dash charger and cable to see if they do work?
Check if there is no dust buildup.
I had the same issue. Tried a new charger and new cable with no success. I had to replace the charging port on my OP5 to solve it.
OP5 is quite old now. Charging port has suffered enough wear and tear. I'm facing the same issue with dash charging, while the same charger is charging my OP6 with dash speed, it fails to dash charge my OP5. I have to plug in with force and keep the phone in one position to let it charge otherwise it stops charging at all
Issue is surely with the charging port, just get it replaced.
If your phone is still under warranty please visit a service center or contact oneplus.com/support.
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by Leo X., 2020-12-03