Ordering the local subsemigroups of a semigroup.
Orders of subsemigroups of Tn.
Ordres maximaux
Orthodox bands of modules
Orthodox congruences on regular semigroups.
Orthodox semidirect products and wreath products of monoids.
Orthodox semigroups.
Orthodox semigroups whose idempotents satisfy a certain identity.
Orthodox semigroups with complemented congruence lattices.
Orthodox -semigroups.
O-simple strictly regular semigroups.
-hypergoupes
Paramedial groupoids
Partial Abelian H_u-Monoids
Partial groupoids with some associativity conditions
Partial orders and their semigroups of closed relations.
Partial orders, closed relations and their automorphisms.
Partial orders in semigroups and semirings of right quotients.
Partially abelian squarefree words
Partially simple semigroups