Unfortunately, under the OSH Act, OSHA may propose penalties of up to $7,000 for other-than-serious violations, but is not required to seek any penalty for them. And - in many instances, OSHA does not.
If OSHA determines that there is a "knowing, deliberate and intentional" effort by the employer to avoid recording occupational injuries and illnesses, resulting in significant underrecording, it may cite the employer for a willful violation. OSHA may also cite on an instance-by-instance basis -- a very expensive proposition.
In working with OSH on this one, you should get all your ducks in a row to show them that the failure to report was not intentional. Showing that the employer did report other similar incidents would help. Showing them that you have procedures (then or now) to make sure all recordable injuries and illnesses are being entered properly on their OSHA 300 logs may also weigh in your favor. An effective way to make such a system is to check entries against other personnel and medical records, such as workers compensation reports of injuries, for consistency. In addition, logs should be checked to ensure that they are complete and all required information has been included.