[6][7] He was an assistant professor at MIT from 1970 to 1972, but then left academia for industry, returning in 1988 as a member of the UMass/Boston faculty.
O'Neil published the algorithms of the bitmap indices he found working in the CCA Model 204 DBMS in the mid-1980s, and implemented B-tree for that database.
[11] O’Neil invented the Log-Structured Merge Tree (LSM Tree) along with Dieter Gawlick and Edward Cheng in 1991 while spending the summer at Gawlick's database research group at Digital Equipment Corporation in California.
The resulting paper,[12] published in 1996, also included a performance analysis by Elizabeth O'Neil.
This access method supports very fast inserts without hobbling lookup times, and now underlies many NoSQL data stores, such as Bigtable, HBase, LevelDB, SQLite4,[13] Tarantool,[14] RocksDB, WiredTiger,[15] Apache Cassandra, InfluxDB,[16] and ScyllaDB.