You are welcome to use addGeneIDs in ChIPpeakAnno package to get OMIDs for a list of gene IDs by setting IDs2Add = c("omim"). Currently, 6 different type of input gene IDs are supported, i.e., ensemble_gene_id, entrez_id, gene_symbol, gene_alias or refseq_id. For more informations, please type ?addGeneIDs in a R session after load the package by library(ChIPpeakAnno).