© 2019 Societe Mathematique de France. Tous droits reserves - We describe additive (unstable) operations from a theory A* obtained from the Levine-Morel algebraic cobordism by change of coefficients to any oriented cohomology theory B* (over a field of characteristic zero). We prove that there is 1-to-1 correspondence between operations An ! Bm and families of homomorphisms An ((P°°) x r ) ! B ™ ((P 1 ) X R ) satisfying certain simple properties. This provides an effective tool of constructing such operations. As an application, we prove that (unstable) additive operations in algebraic cobordism are in 1-to-1 correspondence with the L ®Z Q-linear combinations of Landweber-Novikov operations which take integral values on the products of projective spaces. Furthermore, the stable operations are precisely the L-linear combinations of the Landweber-Novikov operations. We also show that multiplicative operations A* ! B* are in 1-to-1 correspondence with the morphisms of the respective formal group laws. We construct in¬ tegral Adams operations in algebraic cobordism, and all theories obtained from it by change of coef¬ ficients, extending the classical Adams operations in algebraic K-theory. We also construct symmetric operations and Steenrod operations (a la T. tom Dieck) in algebraic cobordism for all primes. (Only symmetric operations for the prime 2 were previously known to exist.) Finally, we prove the Riemann-Roch Theorem for additive operations which extends the multiplicative case done in [18].