Bring your program into OBS as a Browser Source

TCast serves its program as a web page, so any copy of OBS can pull it in as a Browser Source. This works whether OBS is on the same Mac as TCast or on another computer on your network. It carries video and audio, and there's nothing to install in OBS.

The sidebar's Display Output group with the OBS — Browser Source row

What you'll need

OBS Studio.
OBS on the same Mac, or another computer on the same network.

Get the output URL

TCast shows the URL in two places:

From the sidebar

Under Display Output, use the OBS — Browser Source row. The Connect to OBS panel there lists the steps and the URL, with a Copy button.

From Settings

Open Settings (⌘,) and select the Output pane.
Under OBS Browser Source, click Copy next to Output URL.

Settings ▸ Output ▸ OBS Browser Source, with the Output URL and its Copy button

The OBS — Browser Source row shows LINK once OBS is connected, and OFF when nothing is pulling the feed.

Add it in OBS

In Sources, click + ▸ Browser.
Paste the Output URL into the URL field.
Set Width, Height, and FPS to match your stage, for example 1920 × 1080 @ 60 (Settings ▸ Output ▸ Stage). A mismatch here is the usual cause of a scaled or cropped picture.
Tick Control audio via OBS on the source so OBS handles the program audio cleanly.
Click OK.

Your program appears in the scene, in sync, with audio.

What gets sent

The program, exactly what's on air, video and audio together.
The picture is rendered at the Width/Height/FPS you set on the source, so keep those matched to Settings ▸ Output ▸ Stage.

Browser Source or something else?

Browser Source is the right call when you want audio in the same source, or when OBS is on another machine. If OBS is on this Mac and you mainly want video, Syphon is a lighter path. Also, it is generally more reliable, and there are other ways to send audio. The documentation index has the full comparison.

Troubleshooting

The row says OFF and OBS shows nothing

Check that the URL is pasted exactly. Use Copy rather than retyping.
On another computer, make sure it's on the same network and that its firewall isn't blocking the connection.

The picture is scaled, cropped, or blurry

Set the browser source Width/Height/FPS to match Settings ▸ Output ▸ Stage.

There's no audio

Enable Control audio via OBS on the browser source. Without it, OBS may route the audio to the monitor only, or not at all.

The picture is black

Check something is actually on air in TCast. The feed shows whatever the program is showing.