OMNI is the library's 'discovery layer', meaning it is a single search portal that can be used for finding a variety of resources, including books, newspaper and journal articles, films, etc. There are some databases, including ones on the EBSCOHost platform (Academic Search Elite, America: History and LIfe, Historical Abstracts, and others) that are not included in OMNI searches. For those databases, you will need to search them directly.
You can search it directly on the library's home page. To learn more about using OMNI see the OMNI guide.