for UML it seems mermaid is indeed good enough, I used mermaid but never used it for OOD UML, after a quick read it looks very simple and straightforward for UM

for UML it seems mermaid is indeed good enough, I used mermaid but never used it for OOD UML, after a quick read it looks very simple and straightforward for UML comparing to plantuml, especially you can embed mermaid right into markdown as text