One of the many new features of OpenBSD 6.3 is the possibility to dump USB traffic to userland via bpf(4). This can be done with tcpdump(8) by specifying a USB bus as interface:

# tcpdump -Xx -i usb0
tcpdump: listening on usb0, link-type USBPCAP
12:28:03.317945 bus 0 < addr 1: ep1 intr 2
  0000: 0400                                     ..

12:28:03.318018 bus 0 > addr 1: ep0 ctrl 8
  0000: 00a3 0000 0002 0004 00                   .........                     
[...]