The shortest possible answer: David Leavitt created what he originally called the Olde English Bulldogge in 1971. In 2004 he renamed his line the "Leavitt Bulldog" and stopped calling it an OEB. The Olde English Bulldogge name and the broader breed continued under other registries (OBBA, IOEBA, OEBKC) with breeders who used Leavitt's foundation stock, dogs from other lines, or both.
So Leavitt Bulldogs and Olde English Bulldogges share an origin but are now recognized as separate breeds by separate registries. Most Leavitt Bulldogs would qualify as OEBs structurally. Not every OEB qualifies as a Leavitt Bulldog - the LBA only registers dogs with documented Leavitt-foundation lineage.
The split. Around 2005–2008, breeders who wanted to expand outside of Leavitt's strict foundation rules organized OBBA, IOEBA, and OEBKC. Leavitt's response was to formalize the LBA and reserve "Leavitt Bulldog" as a separate breed name.
Appearance and structure
For most buyers walking past, you cannot tell a Leavitt Bulldog from a Leavitt-foundation OEB. Same size range, same head proportions, same coat colors. Both come in the 50–80 lb range, both have functional muzzles, both have the working-bulldog look.
OEBs from non-Leavitt foundation lines (many OBBA and IOEBA dogs) can vary slightly more - a bit more size variance, more color variance, occasional structural differences depending on what the breeder was selecting for.
Health
Effectively the same. Both populations come from the same reconstructed working bulldog program and share most of the same health profile: generally good airways, generally free-whelping, hip risk, occasional cherry eye, occasional allergies.
Registries and dual-registration
Leavitt Bulldog Association (LBA): Strict on Leavitt-foundation lineage. Tighter approval process for studs and litters. Smaller eligible breeding pool.
OBBA, IOEBA, OEBKC, UKC: Recognize OEBs more broadly, with each registry having its own rules about which lines qualify.
UKC accepts LBA papers for OEB single-registration. OBBA accepts LBA papers for dual-registration. Full registry comparison.
Which one to pick
For most buyers, the answer is "whichever is closer to home and has a litter coming." The structural and health differences between a typical LBA-registered Leavitt Bulldog and a typical OBBA-registered OEB are smaller than the differences between two random breeders within either registry.
What matters more than the registry name on the papers is: the breeder you buy from, the parents you can verify, and the contract you sign. Pick the breeder first.