what map should I veto when playing as the OST?
No map is unwinnable on before I say anything, But anyway.
Semoiskiy Summer forces you to spread yourself to thin,youre forced to defend your fuel/muni and lose mid(which is a death sentence),or if you focus mid you cant really defend your fuel well. Essentially a free win for a good allied player. (unless you go CAS full meta of course

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Semoskiy winter,the spreading yourself thin part is fixed somewhat from the summer version thanks to better fuel point placement,but the deep snow is killer. Veto this or Khlodny winter. (Or bystraya

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Next two are basically no brainer veto's.
stalingrad and la gleize. because those are almost purely mid range/close range maps. Not much more needs to be said.