Stands for One Rich Asshole Called Larry Ellison, not officially but I like to think it is. It’s a corporation built around a database engine.
They have a well deserved terrible reputation based on their commercial practices, including but not exhaustive, shipping full fledged software with functionality locked behind paywalls, buying and demolishing established open source companies/projects, suing the shit out of their customers for license violation (see above), price gouging their customers who often have no other choice than to run their products.
The engine itself is nice and reliable but the business practices of Oracle drives a lot of companies to settle for the competition, at least, those who can afford to leave.