SQLAlchemy のfuncは、任意の Oracle 関数を属性として受け入れます。
from sqlalchemy import func
campaign_id = sometable.c.ad_campaign_id
day = func.to_date(sometable.c.day, 'YYYY-MM-DD') # Is this what you need?
spent = func.nvl(sometable.c.spent, 0)
clicks = sometable.c.clicks
cols = [campaign_id, day, spent, clicks]
# Feel free to skip, this is just so you get the labels you want
labels = "campaign_id day spent clicks".split()
col_label = zip(cols, labels)
labelled = [c.label(l) for (c,l) in col_label]
q = session.query(*labelled) # or use *cols if you don't care about labels
q.all()