To be compatible with all Spacemacs bindings, please refer to Conventions. In brief, SPC o is reserved for user custom bindings in global-map, and SPC m o in major modes.

Key sequences are bound to commands in Emacs in various keymaps. The most basic map is the global-map. Setting a key binding in the global-map is achieved with the function global-set-key. Example to bind a key to the command forward-char: 

(global-set-key (kbd "C-]") 'forward-char)

The kbd macro accepts a string describing a key sequence. The global-map is often shadowed by other maps. For example, evil-mode defines keymaps that target states (or modes in vim terminology). Here is an example that creates the same binding as above but only in insert state (define-key is a built-in function. Evil-mode has its own functions for defining keys). 

(define-key evil-insert-state-map (kbd "C-]") 'forward-char)

Perhaps most importantly for Spacemacs is the use of the bind-map package to bind keys behind a leader key. This is where most of the Spacemacs bindings live. Binding keys behind the leader key is achieved with the functions spacemacs/set-leader-keys and spacemacs/set-leader-keys-for-major-mode, example: 

(spacemacs/set-leader-keys "C-]" 'forward-char)
(spacemacs/set-leader-keys-for-major-mode 'emacs-lisp-mode "C-]" 'forward-char)

These functions use a macro like kbd to translate the key sequences for you. The second function, spacemacs/set-leader-keys-for-major-mode, binds the key only in the specified mode. The second key binding is active only when the major mode is emacs-lisp.

Finally, one should be aware of prefix keys. Essentially, all keymaps can be nested. Nested keymaps are used extensively in spacemacs, and in vanilla Emacs for that matter. For example, SPC a points to key bindings for "applications", like SPC a c for calc-dispatch. Nesting bindings is easy. 

(spacemacs/declare-prefix "o" "custom")
(spacemacs/set-leader-keys "oc" 'my-custom-command)

The first line declares SPC o to be a prefix and the second binds the key sequence SPC oc to the corresponding command. The first line is actually unnecessary to create the prefix, but it will give your new prefix a name that key-discovery tools can use (e.g., which-key).

Example to create binding in major mode: 

(spacemacs/declare-prefix-for-mode 'org-mode "mo" "custom")
(spacemacs/set-leader-keys-for-major-mode 'org-mode "oi" 'org-id-get-create)

This would add binding as , oi and SPC moi (note that the "m" in the prefix declaration must be include).

There is much more to say about bindings keys, but these are the basics. Keys can be bound in your ~/.spacemacs file or in individual layers.