I played in an online OSR group for about six years, refereeing for about two and a half of them. Since everyone is shifting their games online to avoid spreading coronavirus infections, I thought I would share some resources and suggestions to make that an easy experience. As always, all of this advice comes with the caveat to do it with charm and grace and not robotically.
I think Discord is the best videoconferencing software for gaming at the moment. Discord allows video chat for up to nine people via group chat with no time limit; asynchronous messaging as either a group chat or a private server; the upload of images under 10 MB, and is less of a RAM hog than Skype. You can also load in a dicebot (the one my current group is using is DiceParser) to handle dice rolling.
I strongly recommend downloading the app instead of running it in your browser, since browsers' RAM usage surges and ebbs unexpectedly, which leads to very uneven video quality, especially when you navigate away from the tab that the video conferencing is running in. The app is much more consistent.
To get the best experience from Discord, I recommend you start about half an hour early for the first session to properly set things up. You should encourage people to shift to "Push to Talk" in their "Voice and Video" settings. This will mean that they're only transmitting purposefully and will eradicate the majority of problems with echoing and intrusive background sounds. I recommend keybinding the "Push to Talk" to an arrow key, since it won't interfere with typing text.
If people don't or won't use Push to Talk, then encourage them to practice good "mute etiquette" by muting their microphones when they're not speaking or getting ready to speak.
The biggest challenge with any online system is reducing cross chatter, which will cause most computer speakers to spit out gibberish and cause slower computers to lag. A related challenge is the pauses where a person has stopped talking and everyone waits a moment then suddenly starts talking at once because the nonlinguistic conversational turn-taking cues are suppressed in video calls. Push to Talk with help a bit, but you can also do a few things to reduce how often this happens.
My suggestions are to adopt gestures that:
1) People can use to indicate that they want to talk. Raising a hand and holding it up until called on by referee works well. Discourage people from flopping their hands up rapidly once hoping that you'll notice. Once the players are familiar with this, you can encourage them to actively "hand off" to the next person they see whose hand is raised.
2) Indicate disagreement or agreement with some proposition. Encourage players to ask simple yes or no questions instead of open-ended, convoluted questions that require several assumptions to hold. Then do a simple roll call and have people give a thumbs up or thumbs down indicating their assent or dissent.
3) Indicate that they're done talking or finishing up, or that you, the referee, want them to bring their comments to a close. I use the OK symbol or sometimes a horizontal hand slice at face-level (I know this as a "cut it off gesture" but these things are so regional).
The smoother you can make all of this, the easier people will find it to pay attention to what's going on and to follow the flow of discussion. The harder it is to follow a given discussion, the easier it is to drift off or become distracted.
Speed is less important than clarity and focus. I recommend supplementing the conversational component by writing out key decisions to be made, or resolutions that have been decided, in the text channel, and having people who have trouble speaking up write their responses there. Rather than handle it yourself, per se, I would recommend appointing either the Caller or the Notetaker to facilitate these discussions and bring them to the attention of the video conference as they think appropriate.
I recommend using the text chat element frequently. When people cast a spell that "lasts 24 hours" and like, have them type that fact into the text channel along with the estimated time of day they cast it. If the party develops a multi-step plan, have them write out the steps in the text channel. Use the text channel as a prosthetic memory so that you don't have to remember all of this stuff.
I also recommend using a digital whiteboard when you play. After trying a few options, I settled on Miro (it used to be called Realtimeboard). You can get a free account, but I actually pay for one that gives me extra space (I use it for work stuff and personal projects as well as gaming).
I like Miro because I think it's got the best and broadest set of tools for organising information, and because it gives you a lot of storage (especially with a paid account). I drag and drop images from around the web to create visual collages, and I toss up pdfs of errata documents, world write-ups, and the like.
With Miro, you can share links where people can only view but not edit the board, or you can invite them using their email address to the board which allows editing even if they don't have an account. I recommend using the latter, and make sure to set it up beforehand. Be warned that boards with lots of stuff that have multiple people logging in at once will be slightly slower to load than bare boards with only one or two people.
To use a digital whiteboard effectively for gaming, you should break up sections by function. I use frames for each collection of related items to aid navigation. I recommend at minimum the following sections:
1) A resources section that's mostly locked down so people can't accidentally move or delete stuff in it. These are your static images (e.g. the map of the campaign setting), errata documents, a document with the key stats for the PCs (like their saving throws), etc. Use Miro's "lock" function to prevent people from deleting, duplicating, etc. these pieces.
2) A "working files" section. This is stuff that you'll change or grab maybe once or twice a session but will need more than once. This is where you place comments or notes containing information the PCs discover about the world or NPCs, dump icons for enemies and PCs for battle map uses, and stick other reusable elements you might need.
3) A central working area where you'll be drawing battle maps, diagrams, and illustrations, moving icons around, sticking up post-its with NPC names during interactions, etc. This is where PCs should be drawing things and making their own notes before they're moved over into a more enduring section.
The real key to this is to use the whiteboard as much as possible to organise the presentation of information so people can understand things visually instead of verbally. This reduces the strain on your audio channel's bottleneck, gives people a different way to focus in on what's going on in the game, and lets you present information that might take a lot of speech to convey clearly, but can be conveyed very simply visually.
If you're just using it for combat, then two common outcomes are that in-between combats people will log out of it and lose their ability to navigate back to the link, or they will keep the chat program maximised on screen and only switch over to the whiteboard slowly and when explicitly told to. I recommend two practices to encourage more fluid switching between chat and whiteboard programs.
1) As referee, share your screen with the whiteboard on it directly into the chat to call PCs attention to it during deliberations, but not during combat. This lets them continue to use the visual signals established above to handle their deliberation, while also allowing you to focus them on the core elements you want to inform that deliberation (literally, by moving your screen to focus in on those elements).
If PCs want to focus in on something that's not on your shared screen during deliberations, encourage them to share their screen of the whiteboard while they talk (and possibly draw or type something out). It will help them capture the attention of other players.
2) Have them shift over to the whiteboard completely for combat. Start off each round with a call to attention for everyone who has drifted off since they last acted, allow them to briefly discuss tactics for the round, and then go through matters in a strict initiative order with the expectation that they won't interrupt one another. Encourage them to communicate with one another via sticky notes on the board, rather than talking out of turn.
Once again, this is the sort of thing that setting expectations at the start, consistently enforcing them, and investing a bit of time to build player skill and familiarity with these methods and the digital tools will make the whole experience much better for everyone.
I hope this is helpful in assisting you to play adventure games online. Good luck!