If you try to use sum() directly in raster::aggregate() and have NA values, you’ll get NA as a result. You need to build a tiny function and pass the rm.na=T command to sum(). More succintly:
# Dissolve duplicate geometries and sum OOIP
fm <- raster::aggregate(fm.raw,
by="OilFieldID",
sums=list(list(function(x) sum(x,na.rm=T),
"OOIP_pooltable")))