/usr/include/singular/omalloc/ is in libsingular4-dev-common 1:4.0.3-p3+ds-5.
This file is owned by root:root, with mode 0o755.
..
/usr/include/singular/omalloc/mylimits.h
/usr/include/singular/omalloc/omAllocDecl.h
/usr/include/singular/omalloc/omAllocFunc.h
/usr/include/singular/omalloc/omAllocPrivate.h
/usr/include/singular/omalloc/omAllocSystem.h
/usr/include/singular/omalloc/omBin.h
/usr/include/singular/omalloc/omBinPage.h
/usr/include/singular/omalloc/omDebug.h
/usr/include/singular/omalloc/omDerivedConfig.h
/usr/include/singular/omalloc/omError.h
/usr/include/singular/omalloc/omGetBackTrace.h
/usr/include/singular/omalloc/omInline.h
/usr/include/singular/omalloc/omInlineDecl.h
/usr/include/singular/omalloc/omList.h
/usr/include/singular/omalloc/omMemOps.h
/usr/include/singular/omalloc/omOpts.h
/usr/include/singular/omalloc/omRet2Info.h
/usr/include/singular/omalloc/omStats.h
/usr/include/singular/omalloc/omStructs.h
/usr/include/singular/omalloc/omTables.h
/usr/include/singular/omalloc/omalloc.h
/usr/include/singular/omalloc/omallocClass.h