diff --git a/o2.sh b/o2.sh
index 27fb002484..b9db4fb2c7 100644
--- a/o2.sh
+++ b/o2.sh
@@ -137,7 +137,7 @@ valid_defaults:
 ---
 #!/bin/sh
 export ROOTSYS=$ROOT_ROOT
-
+CMAKE_BUILD_TYPE=DEBUG
 # Making sure people do not have SIMPATH set when they build fairroot.
 # Unfortunately SIMPATH seems to be hardcoded in a bunch of places in
 # fairroot, so this really should be cleaned up in FairRoot itself for
diff --git a/o2physics.sh b/o2physics.sh
index 09b9ef83c3..75e9ef9547 100644
--- a/o2physics.sh
+++ b/o2physics.sh
@@ -18,6 +18,11 @@ incremental_recipe: |
   mkdir -p $INSTALLROOT/etc/modulefiles && rsync -a --delete etc/modulefiles/ $INSTALLROOT/etc/modulefiles
 ---
 #!/bin/sh
+CMAKE_BUILD_TYPE=DEBUG
+if [[ $ALIBUILD_O2PHYSICS_TESTS ]]; then
+  # Impose extra errors.
+  CXXFLAGS="${CXXFLAGS} -Werror -Wno-error=deprecated-declarations"
+fi
 
 # When O2 is built against Gandiva (from Arrow), then we need to use
 # -DLLVM_ROOT=$CLANG_ROOT, since O2's CMake calls into Gandiva's