Your highest-yield written exam study, based purely on how much each exam contributes to the overall unit mark, should be:

Priority ranking by exam sitting

PriorityExamOverall unit weightingWhy it matters
1Combined Prosthodontics / RPD / MIR / RETT exam15%Biggest single exam-period mark. Strongest “study → weight” return.
2Combined OM + OFP exam/OSCE10%Equal second highest exam-period weighting.
2Combined CDP + Perio exam10%Equal second highest exam-period weighting.
4Oral Surgery final exam/OSCE5%Lower weighting, but still a failed component.
4Paeds final exam5%Same as OS.
4Ortho final exam5%Same as OS and Paeds.

These exam-period assessments add up to 50% of the overall unit mark, and all are failed components/barriers, so you cannot ignore the 5% exams.

Priority ranking by actual content weight inside combined exams

PriorityContent areaEffective exam-period weighting
1Prosthodontics: RPD + MIR / indirect restorations11.25%
2CDP content7.5%
3OM5%
3OFP5%
3Oral Surgery5%
3Paeds5%
3Ortho5%
8RETT3.75%
9Perio content in the CDP/Perio exam2.5%

Best study-time allocation

Assuming equal difficulty and equal current confidence, I’d allocate your written exam study time roughly like this:

AreaSuggested time share
Prosthodontics/RPD/MIR22–25%
CDP15%
OM10%
OFP10%
OS10%
Paeds10%
Ortho10%
RETT7–8%
Perio5%

Practical takeaway

The best study-to-weight ratio is:

Prosthodontics/RPD/MIR > CDP > OM/OFP/OS/Paeds/Ortho > RETT > Perio

But because each assessment is a failed component, use this rule:

First get every exam to safe-pass level, then pour extra time into Prosthodontics/RPD/MIR and CDP, because those are where high marks move your overall unit grade the most.