What about something like:
sage: t = var('t')
sage: Omega = function('Omega')
sage: quanti = Omega(t)*cos(t) + diff(Omega(t), t)
sage: quanti
Omega(t)*cos(t) + diff(Omega(t), t)
sage: Omega0 = var('Omega0')
sage: Omega_const(t) = Omega0
sage: quanti.substitute_function(Omega, Omega_const)
Omega0*cos(t)
Eric.
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