Can we do geo thermal on Oahu?

Likely yes, but one must drill deep.  It remains marginally economic to do so now but certainly not in the future.

Can we power the entire Island

The entire state…

What about Molokai, Lanai and other Islands

Wherever there is volcanism there is geothermal potential. One manifestation of volcanism are thermal (hot) springsl:  Oahu -- in Kalama Valley (paved over by Henry J . Kaiser) = Ko’olau volcano;  Molokai -- reputed to be near Papohaku Beach or thereabouts = Mauna Loa volcano;  Lanai -- none that I know of;  Maui -- mauka of Lahaina = West Maui volcano; perhaps near La Perouse Bay = Haleakala volcano; Big Is. — hot springs galore… 

Plus Kilauea continues to erupt, Mauna Loa has been threatening new eruptive activity on-and-off for the past 2 decades, Hualalai -- erupted last in 1800/1801, Kohala — seems quiet, but Loihi seems plenty active.  As for Kauai, it may have erupted in the 1950’s on the seafloor in the channel separating it from Oahu’s Kaena volcano.

What is keeping us from doing it?

Urgency is not quite here yet… it will come.

What steps are needed to do Georthermal on Oahu

The technology is highly developed all over the planet (Greece, Italy, Poland, Hungary, Turkey, Iceland, New Zealand, Japan, and more)

Is it dangerous healthwise to do geothermal in Hawaii

Current technologies such as used in Pahoa are sealed systems that do not release subsurface chemicals nor contaminate the ground water.  Sulfides and carbon dioxide were released decades ago at the Puna facilities but trace amounts are trace amounts, and in comparison to natural sources such as fumaroles or eruptions what was released was negligible.  H2S once was a problem with the old technology, which oxidized to produce really stinky stuff, but no longer with the new technology.  These are minor straws — silly stuff -- used to discredit perhaps the most negligibly-polluting energy source on this planet … a gift from Pele (or from 'Aila’au depending upon your perspective of the last phase of the last eruption).

What would you like the people of Hawaii to do to prepare for the future

Consider power generation from geothermal, OTEC, abyssal tidal currents over deep sea-floors of channels between islands, and solar (every rooftop in Hawaii should be paved with solar panels)