As a parent I try to say yes as much as possible. I look for ways to say yes.  However, you cannot avoid the need to say no. As Otto is crawling, climbing and reaching he is hearing it more than ever. He is probably hearing it a little too much. I re-read some of my Montessori texts and Montessori from the Start has the best description of how to say no. 

It's important to note that at first infants don't know what no means. Saying no, repeating it and getting louder or more urgent doesn't always work. At this age, you physically need to show your child what no means. This could be as simple as removing the forbidden item from the child's reach, line of sight or redirecting your child. 

Polk Lillard and Lillard Jessen in Montessori from the Start refer to redirecting the child in a respectful manner. 

If you child crawls or walks rapidly away from you, try not to swoop her up from behind into your arms. Instead, go around in front of her and stop her motion by facing her. You are then able to redirect her energies in a decisive and, at the same time respectful manner. 

I often see parents (I have done this myself) treat their child like this, picking them up from behind, without any warning to redirect them. 

By redirecting your child in this way, you are showing her that she is not a "lump of clay" to you; she is a developing person. You are respecting her as someone who must one day "turn herself around" and not expect a "rescue from behind".  

Another tip from Montessori from the Start. Be consistent.

Redirect her behaviour every single time that that behaviour is inappropriate. Do not think that maybe this once you can let something go. You cannot. The child can only internalise firmness and an understanding of finality and limits in life from your consistency and your confident manner in helping her. 

And this wonderful quote:

...the adult's role is to "teach children limits with love or the world will teach them without it."

This is the advice I need right now. Limits with love. 

What does The Montessori Toddler say about this topic?

Pick them up, "No I can't let you touch that". And remove them from the area. 

We may need to continue to repeat this if they keep going back. In this case I would look to see if I can change the environment so they cannot keep getting into danger. Place a box over the power outlet, move a couch in front of wires, or remove a glass cabinet to a room where we can lock the door.

Also found in The Montessori Toddler:

If you say it, mean it. If you mean it, follow through with kind and firm action. - Jane Nelson, Positive Discipline, The First Three Years.