We recommend 2 possibilities (see below).

On the same side of the river as the conference campus (hotels close to campus):

(Possibility 1) Hotels very close to the conference campus (participants on their own; requires just taking a tram; very little walking):

Almada Business Hotel (****)
Rua Abel Salazar, nº.9/9B, 2805-313 Almada

(Possibility 2) Hotels close to the conference campus (a bus will pick up participants early in the morning and take them back late in the afternoon; no walking):

In case you would like to register for the free daily bus shuttle,
please send a message to iccopt@campus.fct.unl.pt.

Meliá Aldeia dos Capuchos (****) (by a golf course)
Largo Aldeia dos Capuchos, 2825-017 Costa de Caparica

Hotel Costa da Caparica (****)  (at the beach)
Av. General Humberto Delgado, 47, 2829-506 Costa de Caparica

There are many other choices for hotels in downtown Lisbon, relatively close to campus, on the opposite side of the river from the conference campus. Here participants are on their own.

For instance, participants stay in hotels (such as Lx Boutique Hotel and Hotel Borges) walking distance to a ferry station (Cais do Sodré), then take a ferry to go across the river, and then a tram to the conference campus.
 
Besides the above ferry connection, there is also a train connection (Fertagus) from the Entrecampos station to the tram station that goes to the conference campus. Participants may also choose hotels close (walking distance) to the Entrecampos train station or take underground wherever they are in Lisbon to this train station. A couple of possibilities are Holiday Inn Lisbon-Continental Hotel and Vip Executive Barcelona Hotel.