When using the command: op <playername>, it will only temporarily op a player until they are disconnected from the server. 

I noticed that my server could not find an ops.json file and so I created my own using the format : 

[

{ "permission": "operator", "xuid": "xxxxx"}

,

{ "permission": "operator", "xuid": "xxxxx" }

]

I then manually added the player(s) I wanted to op by going into ops.json, which works.

I am still finding that using the op command does not add a player to the ops.json.