Similar across manufacture of SOIC8 and SO8, most difference of the 2 would be mostly the leads width from the IC itself. SO8 can havea slightly wider foot print per pin over the SOIC8 with similar between pin gap space per row, they show to fit the same board config.
Overall an SOIC8 would fit a SO8 seating plane as the pins would show slight spare spacing on each side per pin, and an SO8 could show the pins slightly going over the lead pattern edge for an SOIC8 lead plane when following absolute board printing layout seating plane per chip type. These are in millimeters of differences.
:Example: typical range.
0.51mm, 0.0200 inch for SOIC8 Lead pattern width per pin
0.72mm, 0.0283 inch for SO8 Lead pattern width per pin.
:Each pins avg width:
----------------------------------------- :Edited the P in Per displayed as an emoticon character:
per pin space: Center line per pin,
SOIC8 1.27mm, 0.0500 inch center line on pins (middle of pin per 2) :each 2 pins centers are 1.27mm avg from each other:
SO8 1.27mm, 0.0500 inch center line on pins. same as above,
The IC's will interchange in practically all cases if the lead numbers and as initial measurements depending on the board layout permits. *some companies trim as much as possible on PCB layout producing quite small seating points regardless*
SO8 have slightly larger seating plane averaging typical.