Steps 1 to 8 are for Servers 1.7.5 and below. For Servers 1.7.6 and above. Please scroll down and use step 1a
1.7.5 and below
1. Open your control panel
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2. Click on your server
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3. Click Files
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4. Click Config Files
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5. Click Operators
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6. Add user name (one per line) in the box
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7. Click Save
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8. Restart your server
1.7.6 and above
1a. You can add from the multicraft control panel console using: op <playername>
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Here is a link to some OP commands.