The Onpelus 2's charger is not a dash charger and has the Ampere rating of 2A, and has support for Qualcomm quick charge 2.0. The Xiaomi Redmi 5 plus has support for Qualcomm quick charge 3.0 and ampere rating of 2A is supported. If it was a dash charger, then connecting it to a redmi 5 plus would not be advisable, as dash charger has an ampere rating of 4A, which can decrease the battery life of your Redmi 5 plus.
But in this case, the oneplus 2's charger is Qualcomm 2.0 supported and 2A, and Redmi 5 plus also supports Qualcomm quick charge, hence we get to the conclusion that charging your Xiaomi Redmi 5 plus with Oneplus 2's charger will not be a problem for your device.
Deepak Mathews Koshy