Firstly, an EAN code is a number, so you should only use numbers, not characters. Within openERP, EAN-13 is used (as far as I know).

According to Wikipedia:

An EAN-13 barcode (originally European Article Number, but now renamed International Article Number even though the abbreviation EAN has been retained) is a 13 digit (12 data and 1 check) barcoding standard which is a superset of the original 12-digit Universal Product Code (UPC) system developed in the United States.[1] The EAN-13 barcode is defined by the standards organization GS1.

An EAN code consist of the following parts:

1. GS1 Prefix, the first three digits, indicating the country
2. Company number, consisting of three to eight digits
3. Item reference,consisting of two to six digits
4. Check digit, a single checksum digit

If you want to get/generate EAN codes, you have to download a module for that, if it exists. Or write a module yourself.

In the project I am working, they have a requirement to generate the EAN-13, which should be unique as well. So in the near future I might have a module you can install but that will take some time (2 weeks or more from now on, as this is not a high priority).