As you know, the difference between StringBuffer and StringBuilder is that StringBuffer is implicitly synchronized. But within the OIDImpl constructor, it's not multithreaded, so it's just wasted overhead.
StringBuffer s = new StringBuffer();
StringBuilder s = new StringBuilder();
And actually, for pre-determined appends like this, using a StringBuffer/Builder doesn't really make a difference from the "+" operator, so you can make the code more legible by simply concatenating the values directly:
String s = this.pcClass + oidSeparator + this.oid;