Ingres is a commercially supported, open-source RDBMS that competes with the Oracle Database.
Ingres was first created as a research project at the University of California, Berkeley starting in the early 1970s and ending in the early 1980s. The original code was available at minimal cost under a BSD-type license. Since the mid-1980s, Ingres had spawned a number of commercial database applications, including Sybase, Microsoft SQL Server, NonStop SQL and a number of others. Postgres (Post Ingres), a project which started in the mid-1980s, later evolved into PostgreSQL.
Migration to/from Oracle
Use the Ingres copydb utility (with -c flag) to generate ASCII files of all your tables. Then, use SQL*Loader to load in the data into the Oracle database. Stored procedures, rules etc. will have to be converted by hand as the syntax is different.