What does good OR practice mean, and what can critical systems thinking (CST) do for it? This two-part essay proposes new answers to both questions. It reaches out to the wider community of OR professionals and explains from their perspective what CST is all about and why it matters for good practice. Part 1 first reviews the idea and history of systems thinking in OR, as a basis for properly situating CST within OR. It then offers a comparative, non-partisan account of the two strands of CST, critical systems heuristics and total systems intervention, and identifies their combined potential in an ability to enhance the contextual sophistication of OR. The prevalent but inaccurate notion of the history of OR as a linear evolution from 'hard' to 'soft' and 'critical' systems thinking is replaced by an integrated perspective of OR as applied systems thinking.