Yes, the OPPO Reno5 and OnePlus 9 flash charge are universal.
While both phones support fast charging, the charging protocols and power they use may differ. If two phones have the same charging protocol, and the power is the same, then their flash charger can be common.
To ensure safety and best charging results use the original charger and data cable. If the charging protocol of the two mobile phones is different or the power is different, it is recommended not to mix the charger, so as not to cause the problem of slow charging or unable to charge.
In general, in order to ensure safety and best charging results, it is best to use the original charger and data cable. If you need to use another brand of charger, first check whether its charging protocol and power are compatible with the phone.