  Effective collision strengths for forbidden transitions among the 5
energetically lowest finestructure levels of O II are calculated in the
Breit-Pauli approximation using the R-matrix method. Results are presented for
the electron temperature range 100 to 100 000 K. The accuracy of the
calculations is evaluated via the use of different types of radial orbital sets
and a different configuration expansion basis for the target wavefunctions. A
detailed assessment of previous available data is given, and erroneous results
are highlighted. Our results reconfirm the validity of the original Seaton and
Osterbrock scaling for the optical O II ratio, a matter of some recent
controversy. Finally we present plasma diagnostic diagrams using the best
collision strengths and transition probabilities.
