Before the ruling was given that allowed the oral component of the Torah to be written down, what exactly was permitted and what was not permitted?
More specifically, I would like to know if an individual could write down the oral law or parts of it for his own personal use?
Yes, one could write down parts of the Oral Law for his own personal use.
The Rambam writes this explicitly in his introduction to the Yad:
רבנו הקדוש חיבר המשנה. ומימות משה ועד רבנו הקדוש, לא חיברו חיבור שמלמדין אותו ברבים בתורה שבעל פה; אלא בכל דור ודור, ראש בית דין או נביא שיהיה באותו הדור, כותב לעצמו זיכרון בשמועות ששמע מרבותיו, והוא מלמד על פה ברבים.
Rabbeinu Hakadosh compiled the Mishnah. And from the days of Moshe until Rabbeinu Hakadosh, there was no compilation which was used to teach Oral Torah publicly. Rather, in each generation, the Rosh Beis Din or prophet that was in that generation would write for himself a reminder of the topics he had learned from his teachers, and he would teach from memory in public.
וכן כל אחד ואחד כותב לעצמו כפי כוחו, מביאור התורה ומהלכותיה כמו ששמע, ומדברים שנתחדשו בכל דור ודור, בדינים שלא למדום מפי השמועה אלא במידה משלוש עשרה מידות והסכימו עליהן בית דין הגדול. וכן היה הדבר תמיד, עד רבנו הקדוש
And so each and every individual would write for himself according to his ability, from the explanations of the Torah and the Halachos that he had learned, and from the matters which were innovated in each generation, concerning the matters which were not learned through tradition but rather through one of the 13 attributes and was agreed upon by the Great Court. And so the matter continued, until Rabbeinu Hakadosh.
