Running CM and M Modules with OSE: Beyond Basic Compatibility

The compatibility boundary is procedural

Necrotic Gnome's 2020 article on Companion play says that the BECMI Companion and Master sets are easy to use with OSE. It also explains OSE's Basic/Expert basis and its ordinary advancement boundary at level 14. Read that as permission to add a rules source, rather than evidence that OSE already contains everything in the later sets.

The Companion set supplies dominion and warfare material as well as later character advancement. The Master set continues the rules, including weapon mastery and siege material. A referee should select the additional procedures needed by the chosen adventure before extending the campaign's level range.

The roles in the table follow the CM1 and M1 product records. M1's introduction names the Master set and its level range; printed page 4 supplies the Sea Machine as modifications to War Machine.

CM1: retain the decisions that make rulership matter

A domain is more than a base where the party rests. The choices of rulers and commanders help determine the campaign's direction. If you replace the domain procedures, preserve a way for those choices to affect resources and military outcomes. Choose the domain turn, the records each player must keep and the method of resolving an army encounter. The Companion material is the source to consult for the published framework. If you use a house system, write its relationship to the module's military information before a war begins.

You can borrow a CM1 dungeon or settlement for a lower-level OSE game without importing every dominion procedure. That is an adaptation of selected material. It does not establish that an unmodified party can take on every threat or political responsibility in the full module.

M1: naval warfare does not stop at ship hit points

The Sea Machine passage tells the referee to run naval battles with War Machine modifications. Reading only that page leaves out the base system it modifies. Prepare both the army-resolution procedure and the module's fleet-specific additions.

Character advancement is a separate dependency. M1's stated levels exceed the ordinary OSE range, so decide which progression, high-level spells and combat capabilities the party will use. Do not replace a high-level opponent with an ordinary monster of similar hit dice and assume the scenario remains equivalent.

For an existing level-14 OSE party, the choices are to add a compatible advancement framework or redesign the adventure for that party. Neither choice is automatic. Make the decision before assigning encounters, and check how the party will reach the adventure's unusual destinations and participate in its naval campaign.

A worksheet for the next CM or M module

1. Record the module's stated character levels and rules prerequisites from its introduction.
2. List each procedure it invokes: character combat, mass combat, domains, travel or any special resolution. Put a source beside each one.
3. Identify the spells and abilities the adventure assumes. Check the actual approved character sheets against those assumptions.
4. Prepare one sample domain turn or fleet battle before the session. Use the module's own data so the exercise exposes missing definitions.
5. Document substitutions. Keep the result visible to players whose decisions will use it.

The CM or M prefix is a reading cue, not a complete dependency list. Another module may concentrate on a different part of the higher-level game. For Gazetteer character and setting material, use the separate Mystara regional checklist.