Input these in the "Secrets" section, in order to access the levels of Half Life : Blue Shift.

Password - Effect:

Physics is Beautiful - A Leap Of Faith
Combat and Trains - Captive Freight
Red is Scary - Duty Calls
Barney visits Xen - Focal Point
Barney goes to Work - Insecurity
Power is Progress - Power Struggle

Enable OTIS LOVES DREAMCAST on the Access Codes menu to become invulnerable.

Enter these Passcodes to unlock the corresponding level

Password - Effect:

Soldiers eat tacos - "Forget about Freeman!"
Soldiers visit Black Mesa - "We've got hostiles"
Gordon goes to work - Anomalous Materials
Fear finds Gordon - Apprehension
Safe and Sound - Blast Pit
Xenophobia and firepower - Gornarch's Lair
Panic in Xen - Interloper
Scientists at work - Lambda Core
Panic and die - Nihilanth
Files and fear - Office Complex
Trains and rockets - On a rail
Big is Beautiful - Power Up
Regression brings progress - Quesionable Ethics
Recycle or Die - Residue Processing
Combat and death - Surface Tension
Gordon teaches physics - Unforseen Consequences
Fear and Gravity - Xen

Input "Answers Begat Questions" at the Secrets section, by moving the three bars up and down until they spell out this "sentence"

By completing the training mission you will receive this code. Under Options select Enter Codes, "Actions Ignore Silence" wheels make them show the proper phrase and hit A to verify.

In the options screen under codes, enter "DREAMCAST GIVES FIREPOWER" and push a to confirm then go into trigger codes and turn it on. It told me this after I beat the blue shift game.