You'll get less attenuation with a higher mash temp like that. 148-150 would probably have given you a few more points of attenuation but in an oatmeal stout you probably want more mouthfeel anyway.
Your grist bill would offer a few more clues to your attenuation. If you used a lot of c malts or less fermentable malts that would also keep it from going lower.
If you're bottling I'd do as @sneezles61 suggests and take a couple gravity readings to be safe. If you're kegging I'd get it packaged and on gas ASAP to try and offset the effects of oxidation.