Some questions for you so we can offer good opinions as to OG companions:

What ornamental grasses did you/they have in mind? Some types are quite small while some can get 6' across, some 12' tall.

Your parents probably want OG that doesn't need a lot of maintenance in the years to come, so that's something to think about when choosing a OG...

Also, OG have different color hues, often changing during the seasons. The color may affect which companions would be more appealing.

Deb

Good questions. They will probably lean toward green and possibly variegated. I'm thinking they will go with a maidengrass (miscanthus), ravennagrass or zebra grass.

The posts they will want the grasses in front of are probably 5-6 ft tall or more so size is fine.

Any and all suggestions about grasses and companions are welcome. Thanks!

As companions, I'd like to suggest sedums. There are so many to choose from. Some of my favorites are Autumn Joy, Carl, Class Act and Dazzleberry (a low grower). Many sedum change color as the seasons progress and can offer some winter interest as well. They are sun lovers like OG and, once established, quite drought tolerant.

Another choice might be a shorter OG between the big ones. Boulder Blue fescue make interesting spikey mounds.

While my suggestions might not be the answer, I hope this gives you a springboard for ideas.
:-)
Deb