It's not true. OTA+output buffer=OPAMP. The term OTA can be applied to any of diff. stage configuration, e.g. CS with active load, telescopic, folder cascode... Their have large output resistance Rout for increase gain (A=Gm*Rout or A=(Gm*Rout)^2) and their isn't situable for drive resistive and large capacitive loads. That is why their is named as OTA. For drive resistive and large capacitive loads it's necessary to provide output buffer with low output resistance and such circuit is OPAMP.
opamp means V input , V output ,
But ota means V input , I output , G=I/V
So , the difference is opamp 's output is a volatage signal
, but OTA is current for its' high output impendence , so If a OTA is followed
by a buffer and the buffer 's output impendece is low , it become a opamp