作 SAKU, SA, tsuku(ru) - make
勢 SEI, ikio(i) - force, energy, vigor, trend
役 YAKU, EKI - post, service, battle
望 BOU, mou, nozo(mu) - hope
病 BYOU, HEI, ya(mu), yamai - sick, illness, suffer from
All take 大 (DAI, oo(kii) - big) in front to make a word, but only 大勢 uses the "Oo" reading (Oozei); the others are all "Tai/Dai-":
大作 taisaku - a great work
大勢 oozei - many people
大役 taiyaku - an important role
大望 taimou - a great hope
大病 taibyou - serious illness