OMA standalone downloads only the GeneOntology www.geneontology.org/ontology/gene_ontology.obo from the web if needed (i.e. if DoGroupFunctionPrediction in parameter.drw is set to true and the file hasn't been downloaded before). From the error, it seems that you were either not connected to the internet when you started OMA standalone, sitting behind a proxy and curl doesn't know about it, or the gene ontology server was down at that time).
In case your internet connection needs to go through a proxy server, you can set an environment variable http_proxy. see man curl for more details.