Grows hot, pretty shady, dry tropical forest. Needs quick drying media or mount. Taper off in winter (Jan Feb Mar), drier and brighter. Very wet May through October. http://www.weather-and-climate.com/average-monthly-Rainfall-Temperature-...
Backbulbs blog guy says that this plant will go into a funk if the roots are disturbed, so move it root ball and all into its next pot when repotting if it is not mounted.
Orchid web says Your Onc ampliatum, now known as Chelyorchis ampliata, hates wet, soggy roots, it needs a media that will drain freely and dry quickly. In my opinion, fine bark is too water retentive, I would go with a medium bark mix with perlite and charcoal. It enjoys very high light level and during the winter wants to go dry for longer than normal, short of dessicating the leaves of bulbs. I would also with hold fertilzer during the winter and not start fertilizing again until the new growth starts in the spring. Your winter temps are a bit cooler than is normal for its habit but should not be an issue if you follow the reduced watering suggested.
Ray of firstray.com says
This species (now considered to be a rossioglossum) is native to Guatemala, Honduras, El Salvador, Nicaragua, Costa Rica, Panama, Trinidad, Venezuela, Colombia, Ecuador, and Peru. This widely distributed orchid grows on trees in rather open, bright positions in scrub and lower, seasonally dry forests from sea level to 1950 ft.
I find they do really well mounted, growing along side cattleyas, but are able to tolerate brighter conditions, plus other cultural methods (including S/H) as well. Grow them warm, as their native habitat rarely, if ever exposes them to temperatures below 65°F at night.
