Protocol classes in OPM must satisfy three types of constraints: identification constraints (similar to base object classes), protocol expansion constraints, and input-output consistency constraints.
Protocol expansion constraints
are implied by protocol expansion definitions
and regard every pair of protocol classes, 
and 
, where
and 
are specified as component (sub) protocols of
protocol class 
and 
precedes 
:
every pair of related protocols of 
and 
are required
to be either not associated with a generic protocol or associated with
the same protocol of 
.
Input-output consistency constraints are specified in an OPM schema using input-output attribute specifications and regard input-output relationships between protocol classes:
and 
, where
is the generic protocol class of 
,
for each input or output attribute of 
, 
, that is
specified using an input isa 
.
or
output isa 
.
specification,
the 
value for each protocol of 
must be a subset of
the 
value for the related (generic) protocol of 
;
and 
, where
succeeds 
,
for every input attribute of 
, 
, that is related
using a statement involving a
from ... via specification involving an output attribute
of 
, 
, the values of 
and 
for
two related protocols of 
and 
must be related according to the
from ... via specification.