What is the difference between an 'ok' and an 'OK' under the linear solution section of an out file?
The three columns to the right of the residual diagnostics refer to the performance of the linear (inner) solvers. The first column is the average number of iterations the linear solvers attempted to obtain the specified linear equation convergence criteria (within a specified number of iterations). The second column gives the normalized residuals for the solutions to the linear equation. The last column can have one of four entries: *  indicates that there was a numerical floating point exception and this resulted in the failure of the linear solvers. F  indicates that the linear solvers did not reduce the residuals (i.e., the solution was diverging), but the linear solvers may carry on if the divergence is not catastrophic. ok  indicates that the residuals were reduced, but that the degree of reduction did not meet the specified criteria. OK  indicates that the specified convergence criteria for the reduction of residuals was achieved. 

