I had to google OJT. I've never heard it as an acronym before!
I'm not sure that "take on" is the best choice because it leads to an ugly repetition of "on". Even when using the acronym so one doesn't actually say the word twice, people reading it might well say the words in their head rather than use the initials, and it might feel a little odd.
In general, I'm not sure one really does "take on training" (of any kind). One "has" or "does", possibly "undergoes" although I don't like that - it feels like someone is trying to find a somehow more refined word for "does" and feels a little fake.
You could "go through a period of OJT", or "spend X days/weeks in OJT".
Student or Learner