An apartment can be handed over without a written lease. The key question is whether use was agreed in return for payment or whether the owner merely allowed free use without a defined term or defined purpose. For the second situation, section 974 ABGB describes a precarious loan, also called a precarium.
The label does not decide the case. Messages, payments, discussions about duration, the reason for the handover and the parties' conduct must be considered together. The articles on subletting rights in an apartment building and on the end of a head lease concern paid contractual relationships with a different starting point.