CTR_DRBG versus OFB_DRBG As far as I know, in the development of ANSI X9.82, there was consideration of OFB_DRBG, a pseudorandom number generator based on block ciphers like CTR_DRBG, but it appears to have been rejected of left out of the final publication, much like MS_DRBG. Is CTR_DRBG safer than OFB_DRBG? If so, is this because OFB_DRBG would be susceptible to a known-plaintext attack?
No OFB_DRBG isn't directly vulnerable to attacks.

However from a theoretical standpoint there is one point speaking against it and in favor of CTR_DRBG.

Speed. CTR can be parallelized (can encrypt counters in parallel), OFB can't (encrypting state over and over again)

So the main reason is speed I'd guess and of course adding OFB_DRBG wouldn't have added anything to the portfolio, as CTR does the same job, but better.