Using the English language has been prohibited me there in the past.
So I kindly removed my post 33153.
OC has also a tolerance setting in the Application Preferences and it can bridge gaps.
OG will always result in an open form, OC might result in a closed form when geometrically closed.
OG or OC with a larger tolerance will shift your original entities !!
Look very close at what happens.
With a gap, the next startpoint is replaced by the former endpoint.
They are not auto trimmed!
My advice here is to use OC with bridging gaps, and then only for all line segments.
QCAD will then add minute bridges that are perfectly inline with the main entities.
These get simplified as simplification is a fixed default with OC.
IMHO: OG-OC should auto trim, also for arcs.
That is unless there are good reasons not to.
The best practice in QCAD is to trim them pairwise upfront, all individually ... All one on one.