diff --git a/c/cli/mlrcli.c b/c/cli/mlrcli.c
index ab6f885cb..164071676 100644
--- a/c/cli/mlrcli.c
+++ b/c/cli/mlrcli.c
@@ -202,6 +202,8 @@ static char* rebackslash(char* sep) {
 
 #define DEFAULT_JSON_FLATTEN_SEPARATOR ":"
 
+#define DEFAULT_OOSVAR_FLATTEN_SEPARATOR ":"
+
 // ----------------------------------------------------------------
 // The main_usage() function is split out into subroutines in support of the
 // manpage autogenerator.
@@ -594,6 +596,8 @@ cli_opts_t* parse_command_line(int argc, char** argv) {
 	popts->quote_json_values_always       = FALSE;
 	popts->json_flatten_separator         = DEFAULT_JSON_FLATTEN_SEPARATOR;
 
+	popts->oosvar_flatten_separator       = DEFAULT_OOSVAR_FLATTEN_SEPARATOR;
+
 	popts->ofmt              = DEFAULT_OFMT;
 	popts->oquoting          = DEFAULT_OQUOTING;
 
@@ -769,6 +773,11 @@ cli_opts_t* parse_command_line(int argc, char** argv) {
 			popts->json_flatten_separator = cli_sep_from_arg(argv[argi+1], argv[0]);
 			argi++;
 
+		} else if (streq(argv[argi], "--vflatsep")) {
+			check_arg_count(argv, argi, argc, 2);
+			popts->oosvar_flatten_separator = cli_sep_from_arg(argv[argi+1], argv[0]);
+			argi++;
+
 		} else if (streq(argv[argi], "--csv"))      { popts->ifile_fmt = popts->ofile_fmt = "csv";
 		} else if (streq(argv[argi], "--icsv"))     { popts->ifile_fmt = "csv";
 		} else if (streq(argv[argi], "--ocsv"))     { popts->ofile_fmt = "csv";
diff --git a/c/cli/mlrcli.h b/c/cli/mlrcli.h
index afea95c3d..2e3b16ad8 100644
--- a/c/cli/mlrcli.h
+++ b/c/cli/mlrcli.h
@@ -36,6 +36,8 @@ typedef struct _cli_opts_t {
 	int   quote_json_values_always;
 	char* json_flatten_separator;
 
+	char* oosvar_flatten_separator;
+
 	char* ofmt;
 	quoting_t oquoting;
 
diff --git a/c/todo.txt b/c/todo.txt
index 163566b89..9485ee591 100644
--- a/c/todo.txt
+++ b/c/todo.txt
@@ -59,6 +59,8 @@ TOP-OF-LIST SUMMARY
 ----------------------------------------------------------------
 IMPLEMENT:
 
+? people may ask for ingest version of emit ...
+
 ? try to merge emitf and emit
 
 * cli-parameterize the joinchar (:)