The present study explores the effect of in-cylinder generated non-thermal plasma on hydroxyl and soot development. Three optical diagnostics of electronically excited hydroxyl (OH*) chemiluminescence, Planar Laser Induced Fluorescence of OH (OH-PLIF) and Planar Laser Induced Incandescence (PLII) are performed in a single-cylinder optical diesel engine. Plasma was generated using a newly developed Microwave Discharge Igniter (MDI), which has the potential to accentuate the formation of active radical pools as well as suppress soot formation while stimulating soot oxidation. Methyl Decanoate fuel was specifically selected for diagnostics due to its low beam attenuation which is required for OH-PLIF and PLII. While investigating the behaviour of MDI discharge under engine motoring conditions using a single pulse discharge strategy, it was found that plasma-induced OH* signal size and intensity increased with higher in-cylinder pressures albeit with shorter lifetime and lower breakdown consistency. Results also indicated that a decreasing pressure gradient extends the lifetime of plasma-induced OH* signals; an increasing pressure gradient suppresses plasma-induced OH* formation and increases the rate of signal decay. Studies on the effect of MDI plasma breakdown at the start of high temperature reaction when plasma is discharged during the ignition delay phase were also carried out with the single pulse discharge strategy. Despite plasma-induced OH* signals being detected from the motoring experiments, no significant difference in OH* or OH-PLIF signals were observed under fuel injected conditions when compared to the baseline case. Further exploring the effect of MDI breakdown on the soot formation phase and oxidation period, PLII images show that when using the single pulse discharge strategy, soot formation is suppressed resulting in a decrease in soot intensity at earlier crank angles with smaller area distribution at earlier and later crank angles. The significance of this effect is even more prominent with the implementation of multiple pulse discharge strategies, leading to significantly reduced soot over all crank angles.