getting the string of enum

You can implement static method such as fromByte :

public enum OpCode {
... truncated
    public static OpCode fromByte(byte x) {
        for(OpCode oc : values()) {
           if (oc.getValue() == x) {
               return oc;
           }
        }
        throw new IllegalArgumentException("Unknown OpCode value: " + x);
    }
}

And then you can call it like this:

OpCode oc = OpCode.fromByte(x);

Either linear search as Joe C suggest in its comment or use a Map to store OpCode instances by Byte value.

 public static enum OpCode {

    a((byte) 0x0),
    b((byte)0x18),
    c((byte)0x1A);

    private final byte value;

    private final static Map<Byte, OpCode> opCodesByByte;

    static {
          opCodesByByte = new HashMap<>();
          for (OpCode opCode : values()) {
             opCodesByByte.put(opCode.value, opCode);   
          }
    }

   ...    
  }

And also add a static method in OpCode to provide the search method :

public static OpCode of(byte value){
   return opCodesByByte.get(value);
}

Note that in this implementation, it returns null if no matching.

You need to loop the enum:

    byte x = 0x18;
    for (OpCode opCode : OpCode.values()) {
        if (x == opCode.getValue()) {
            System.out.println(opCode);
        }
    }

Alternatively, build a map like this:

    private static Map<Byte, OpCode> valueToOpCode = new HashMap<>();

    static OpCode getOpCode(byte value) {
        return valueToOpCode.get(value);
    }

    public static enum OpCode {
        a((byte) 0x0),
        b((byte)0x18),
        c((byte)0x1A);            

        private byte value; 

        private  OpCode(byte value) {
            this.value = value;
            valueToOpCode.put(value, this);
        }

        byte getValue() {
            return value;
        }

    }

    private void test() {            
        byte x = 0x18;
        System.out.println(getOpCode(x)); // prints 'b'             
    }