The common problem

Many capture cards only capture landscape signals. A normal phone portrait output can become letterboxed, wasting pixels.

Use OBS Portrait Optimized

HDMI Cam outputs a full-frame landscape signal designed to be rotated inside OBS, preserving usable portrait pixels.

Rotate in OBS

In OBS, right click the capture source, choose Transform, then rotate 90 degrees. Fit it to a 1080 x 1920 canvas.

When not to use it

If your capture hardware already accepts portrait capture correctly, Phone Direct Output may be simpler.