It turns out OCRA RFC 6287 defines some specific HOTP variants...

```
   We list the following preferred modes of computation, where * denotes
   the default CryptoFunction:

   o  HOTP-SHA1-4: HOTP with SHA-1 as the hash function for HMAC and a
      dynamic truncation to a 4-digit value; this mode is not
      recommended in the general case, but it can be useful when a very
      short authentication code is needed by an application

   o  HOTP-SHA1-6: HOTP with SHA-1 as the hash function for HMAC and a
      dynamic truncation to a 6-digit value

   o  HOTP-SHA1-8: HOTP with SHA-1 as the hash function for HMAC and a
      dynamic truncation to an 8-digit value

   o  HOTP-SHA256-6: HOTP with SHA-256 as the hash function for HMAC and
      a dynamic truncation to a 6-digit value

   o  HOTP-SHA512-6: HOTP with SHA-512 as the hash function for HMAC and
      a dynamic truncation to a 6-digit value
```

So I guess having this option here is indeed sensible :) And it seems out limit of 8 digits is pretty reasonable too.