Help deciphering caesar cipher
You are actually doing the right thing, however you do not include the "_" as part of the characterset. Assuming the alphabet consists of 27 letters (A-Z and "_"), and using the appended JavaScript script (you can simply paste it into the console), returns one sentence which actually does make sense:

THE_FAILURE_MAY_BE_BOTH_DELIBERATE_AND_CLANDESTINE

var ciphertext = "OC_VAWDGPM_VHWTVX_VXJOCVZ_GDX_MWO_VWIZVYGWIZ_NODI_";

var alphabet = ["A","B","C","D","E","F","G","H","I","J","K","L","M","N","O","P","Q","R","S","T","U","V","W","X","Y","Z","_"];

var rotation = 1;

var cleartext = "";

while (rotation &lt; 27) {
for (var i=0,l=ciphertext.length;i&lt;l;i++) {

  var character = ciphertext[i];

  var index = 0;

  // get the number in the alphabet
  for (var j=0;j&lt;27;j++) {
    if (alphabet[j] === character) {
      index = j;
      break;
    }
  }

  // rotate

  cleartext += alphabet[(index + rotation) % 27];
}

console.log(cleartext);
rotation++;
cleartext = "";
}


EDIT: There is one thing I wanted to add regarding the script above. We are doing $index + rotation \bmod 27$ here because we are trying every possibility. In a strict sense this is another encryption of the ciphertext, only that one rotation must of course result in the cleartext. If $rotation$ is already known and you want to decrypt the cipher using this exact value, you would of course have to calculate $(index - rotation + 27) \bmod 27 $.