Hello! I agree with Tom and I would like to add that "only just" something happened by a very small margin; almost not. So an example would be: "The man's race time was a second less than his his opponent so he won, but only just." So it means to achieve something or to be successful, by a very small margin - that the possibility was there that it could have been a failure. I hope that helps!

In my experience, the most common use of "only just" has to do with time. I only just started doing my homework. (I recently started, with "only" suggesting that I haven't been able to do much yet) We've only just begun. (A title of a well-known song) The other answers describe your example well. (barely, by a small margin)

"Only just" can be used in a few different contexts, depending on which sentence it is used in - it means that the thing that happened *almost* didn't or couldn't happen. In this case, John "only just" got on the bus becuase there were so many people inside that there wouldn't have been enough space for him if there was one extra person inside. Another use for "only just" would be if he was running late for the bus, but when he got to the stop it was still there waiting for him and he managed to get on it - that would also be "he only just made the bus on time"