We apply Leifer-Milner RPO approach to the lambda-calculus, endowed
with lazy and call by value reduction strategies. We show that, 
contrary to process calculi, one can deal directly with the lambda-calculus
syntax and apply Leifer-Milner technique to a category of contexts, provided 
that we work in the framework of weak bisimilarities. However, even in 
the case of the transition system with minimal contexts, the resulting 
bisimilarity is infinitely branching, due to the fact that, in standard 
context categories, parametric rules such as beta-rule can be represented only
by infinitely many ground rules. To overcome this problem, we introduce the 
general notion of second-order context category. We show that, by carrying 
out the RPO construction in this setting, the lazy (call by value) 
observational equivalence can be captured as a weak bisimilarity equivalence
on a finitely branching transition system. This result is achieved 
by considering an encoding of lambda-calculus in Combinatory Logic.