No such thing as perfect.... everything has tolerance.
Let's say your ocxo is very good and has initial frequency offset of .1ppm and for sake of argument the external clock is perfect 10Mhz. 10Mhz, .1ppm works out to be ( .1ppm/1e6*10M) = 1Hz/sec. Remember, that this is a oscillator, so error accumulates. So, every 1 s you have 1 count.
If you used a 8 bit counter, every 256 second your counter will under/overflow, forcing your pll to glitch.
Update: I should mention the .1ppm can be either direction... so the internal ocxo can be faster or slower then 10Mhz... This is even before things like aging and stability is taken into account.