1) http://www.w3.org/TR/2009/REC-owl2-syntax-20091027/#Object_Property_Expressions As one can see from the figure, OWL 2 supports only two kinds of object property expressions. Object properties are the simplest form of object property expressions, and inverse object properties allow for bidirectional navigation in class expressions and axioms. I believe that there should be an explicit statement about what an ObjectPropertyExpression actually is. Suggestion: An ObjectPropertyExpression is an expression specifying a set of ordered tuples where the first and second member of each tuple is are respectively individuals in the ObjectPropertyDomain and ObjectPropertyRange of the ObjectPropertyExpression.
The editors opted for a slightly shorter wording, and modified the data property expressions section for symmetry.