Since Minecraft 1.7.8, there has been a new file format to the ops file.

You can no longer just enter usernames as it must be in a valid JSON format containing the player’s username, UUID and level

You can find your UUID Here

What the file should look like if you have more than 1 op. Make sure you add the comma between entries.
Do not include the [ ] these are to show you were to put your details

[  
   {  
      "uuid":"[USERS UUID]",
      "name":"[USRE NAME]",
      "level":4
   },
   {  
      "uuid":"[USERS UUID]",
      "name":"[USRE NAME]",
      "level":4
   }
]

and if you only have 1

[  
   {  
      "uuid":"[USERS UUID]",
      "name":"[USRE NAME]",
      "level":4
   }
]

As far as we are aware, at this time, there is no function to the level, but leaving it at 4 will guarantee you have access to the commands.

Unless this file is exactly correct, it will reset when the server starts. If this doesn’t work for you, drop us a ticket explaining you followed this guide.