Usually RF will improve up to some point, and then the test (or OOB) accuracy will rattle around a little bit, centered around some final value. RF should never

Usually RF will improve up to some point, and then the test (or OOB) accuracy will rattle around a little bit, centered around some final value. RF should never overfit from having too many trees. Both plots start at 50 trees, so it looks like the 'improve' part of that is over by then, and what you are seeing here is variation due to different trees being slightly better/worse. Incidentally, that means that the variation in both plots is probably not meaningful and that 'best' score of 99.4% at 250 trees is probably basically an outlier.