nat on $ext_if inet from ! $ext_if to any -> ($ext_if)
nat on $ext_if from !self
man pf.conf
doesn't give you the same accurate, OS-specific information, on OpenBSD as it does in FreeBSD's pf.conf(5). That's certainly going to be the go to place to get what you need in the future. I'm going to close the topic as we don't support other operating systems so if you do run into problems please direct them toward the OpenBSD mailing lists.