HBO can be yours on Sling TV starting today if you're willing to pay the extra $15 fee each month. Add that on top of Sling's $20 subscription price and you'll get both the linear, live HBO channel plus a video-on-demand section that's the same sort of thing cable providers offer. Sling says that HBO is available across all devices that support the internet TV service today; those include Roku, Xbox One, Fire TV, iOS, Android, PC, and Mac.
You can stream HBO on three devices at once
Here's where it gets a bit interesting: since you can't use the HBO Go app as a Sling customer (Sling says you shouldn't need to since everything's right in its own app), the company is upping the number of simultaneous streams it's allowing. But only for HBO. Normally Sling TV enforces a single-stream limit, so you're restricted to watching it on one device at all times. But you'll be able to stream HBO on up to three devices simultaneously. This is probably a modest attempt on Dish's part to make up for the lack of proper HBO Go support. Sling says this new multi-stream functionality will be added to customer accounts "on a rolling basis starting today."