flow rate is dependent on if it is a high flow or a low flow device your using in your case it is your ozone generator, with low flow .50 or less is better with high flow 2, 3, 5, 10 l/m can be used dependent upon the configuration, amount of oil, etc...
the bubble size does not matter as mush with oil as compared to water. ozonating water you need a fine diffuser but oil will bond with o3 much easier then water will. thus if you wanted you could simply poke holes in your silicon tubing that is in your oil. do not use a stone diffuser as that will clog up and break down.