The serial port receipt printing in Openbravo POS (version 2.30.2) was implemented for 32 bit Windows. To use it on a 64 bit Windows, you will have to swap the rxtxSerial.dll (under the lib\Windows\i368-mingw32 directory) for the 64 bit version. This is available from the jLog project (thanks!). Since they also provide a corresponding RXTXcomm.jar, it makes sense to swap this as well (under the lib directory).
By the way, if you are using a Star Micronics printer, you will have to configure the virtual serial port under the Star line mode rather than under the ESC/POS mode.