  We give a detailed analysis of the oscillation formula within the context of
the wave packet formalism. Particular attention is made to insure flavor
eigenstate creation in the physical cases (Delta p not equal 0). This
requirement imposes non instantaneous particle creation in all frames. It is
shown that the standard formula is not only exact when the mass wave packets
have the same velocity, but it is a good approximation when minimal slippage
occurs. For more general situations the oscillation formula contains additional
arbitrary parameters, which allows for the unknown form of the wave packet
envelope.
