Ksenia is right, it depends on which case you're going to use. However, there is one thing you should understand: usually there are different endings for feminine and masculine nouns, but that's not the only criterion. The way noun endings change depends not only on their number and gender, but also on the declension (grammatical group) each noun belongs to. In Russian there are three of them. You can find more information here: http://masterrussian.com/aa052000a.shtml
Speaking about your question, the latter is an example of a first declension noun change (Nominative case -> Accusative case). Москва -> Москву, земля -> землю, юноша -> юношу. The first one, however... Well, I'm not sure if there is any feminine noun whose ending changes from (a) to (o). Could you provide an example, please?